diff --git a/src/blenderbim/blenderbim/bim/module/model/opening.py b/src/blenderbim/blenderbim/bim/module/model/opening.py
index 6795a02147..1d55fb305d 100644
--- a/src/blenderbim/blenderbim/bim/module/model/opening.py
+++ b/src/blenderbim/blenderbim/bim/module/model/opening.py
@@ -111,7 +111,7 @@ class FilledOpeningGenerator:
                 # Equivalent to "side Z" for a wall axis, so that stuff like skylights appear on the top.
                 local_position_on_voided_obj.z = layers["offset"] + layers["thickness"]
                 new_matrix.translation.xyz = voided_obj.matrix_world @ local_position_on_voided_obj
-                rotation_matrix = Matrix.Rotation(radians(-90), 4, 'X')
+                rotation_matrix = Matrix.Rotation(radians(-90), 4, "X")
                 new_matrix @= rotation_matrix
 
             filling_obj.matrix_world = new_matrix
@@ -188,7 +188,13 @@ class FilledOpeningGenerator:
                 )
 
     def regenerate_from_type(self, usecase_path, ifc_file, settings):
-        filling = settings["related_object"]
+        relating_type = settings["relating_type"]
+
+        for related_object in settings["related_objects"]:
+            self._regenerate_from_type(related_object)
+
+    def _regenerate_from_type(self, related_object: ifcopenshell.entity_instance) -> None:
+        filling = related_object
         if not getattr(filling, "FillsVoids", None):
             return
 
diff --git a/src/blenderbim/blenderbim/bim/module/model/profile.py b/src/blenderbim/blenderbim/bim/module/model/profile.py
index 8a4bde560d..2fed398cbe 100644
--- a/src/blenderbim/blenderbim/bim/module/model/profile.py
+++ b/src/blenderbim/blenderbim/bim/module/model/profile.py
@@ -96,7 +96,7 @@ class DumbProfileGenerator:
             should_add_representation=False,
             context=self.body_context,
         )
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=self.relating_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=self.relating_type)
 
         material = ifcopenshell.util.element.get_material(element)
         material.CardinalPoint = self.cardinal_point
@@ -211,11 +211,18 @@ class DumbProfileRegenerator:
         return results
 
     def regenerate_from_type(self, usecase_path, ifc_file, settings):
-        obj = tool.Ifc.get_object(settings["related_object"])
-        if not obj or not obj.data or not obj.data.BIMMeshProperties.ifc_definition_id:
+        relating_type = settings["relating_type"]
+
+        new_material = ifcopenshell.util.element.get_material(relating_type)
+        if not new_material or not new_material.is_a("IfcMaterialLayerSet"):
             return
-        new_material = ifcopenshell.util.element.get_material(settings["relating_type"])
-        if not new_material or not new_material.is_a("IfcMaterialProfileSet"):
+
+        for related_object in settings["related_objects"]:
+            self._regenerate_from_type(related_object)
+
+    def _regenerate_from_type(self, related_object: ifcopenshell.entity_instance) -> None:
+        obj = tool.Ifc.get_object(related_object)
+        if not obj or not obj.data or not obj.data.BIMMeshProperties.ifc_definition_id:
             return
         DumbProfileRecalculator().recalculate([obj])
 
diff --git a/src/blenderbim/blenderbim/bim/module/model/slab.py b/src/blenderbim/blenderbim/bim/module/model/slab.py
index de1d44715b..ec23ca101f 100644
--- a/src/blenderbim/blenderbim/bim/module/model/slab.py
+++ b/src/blenderbim/blenderbim/bim/module/model/slab.py
@@ -35,6 +35,7 @@ from math import radians
 from mathutils import Vector, Matrix
 from blenderbim.bim.module.geometry.helper import Helper
 from blenderbim.bim.module.model.decorator import ProfileDecorator
+from typing import Optional
 
 
 def calculate_quantities(usecase_path, ifc_file, settings):
@@ -177,7 +178,7 @@ class DumbSlabGenerator:
             should_add_representation=False,
             context=self.body_context,
         )
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=self.relating_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=self.relating_type)
 
         blenderbim.core.geometry.edit_object_placement(tool.Ifc, tool.Geometry, tool.Surveyor, obj=obj)
         representation = ifcopenshell.api.run(
@@ -250,27 +251,38 @@ class DumbSlabPlaner:
                             self.change_thickness(element, total_thickness)
 
     def regenerate_from_type(self, usecase_path, ifc_file, settings):
-        obj = tool.Ifc.get_object(settings["related_object"])
-        if not obj or not obj.data or not obj.data.BIMMeshProperties.ifc_definition_id:
-            return
-        self.unit_scale = ifcopenshell.util.unit.calculate_unit_scale(ifc_file)
-        new_material = ifcopenshell.util.element.get_material(settings["relating_type"])
+        relating_type = settings["relating_type"]
+
+        new_material = ifcopenshell.util.element.get_material(relating_type)
         if not new_material or not new_material.is_a("IfcMaterialLayerSet"):
             return
+
         parametric = ifcopenshell.util.element.get_psets(settings["relating_type"]).get("EPset_Parametric")
         layer_set_direction = None
         if parametric:
             layer_set_direction = parametric.get("LayerSetDirection", layer_set_direction)
         new_thickness = sum([l.LayerThickness for l in new_material.MaterialLayers])
-        material = ifcopenshell.util.element.get_material(settings["related_object"])
+
+        self.unit_scale = ifcopenshell.util.unit.calculate_unit_scale(ifc_file)
+        for related_object in settings["related_objects"]:
+            self._regenerate_from_type(related_object, layer_set_direction, new_thickness)
+
+    def _regenerate_from_type(
+        self, related_object: ifcopenshell.entity_instance, layer_set_direction: Optional[str], new_thickness: float
+    ) -> None:
+        obj = tool.Ifc.get_object(related_object)
+        if not obj or not obj.data or not obj.data.BIMMeshProperties.ifc_definition_id:
+            return
+
+        material = ifcopenshell.util.element.get_material(related_object)
         if not material or not material.is_a("IfcMaterialLayerSetUsage"):
             return
         if layer_set_direction:
             material.LayerSetDirection = layer_set_direction
         if material.LayerSetDirection == "AXIS3":
-            self.change_thickness(settings["related_object"], new_thickness)
+            self.change_thickness(related_object, new_thickness)
 
-    def change_thickness(self, element, thickness):
+    def change_thickness(self, element: ifcopenshell.entity_instance, thickness: float) -> None:
         body_context = ifcopenshell.util.representation.get_context(tool.Ifc.get(), "Model", "Body", "MODEL_VIEW")
         obj = tool.Ifc.get_object(element)
         if not obj:
diff --git a/src/blenderbim/blenderbim/bim/module/model/wall.py b/src/blenderbim/blenderbim/bim/module/model/wall.py
index 3463cb5f78..78e8b4c579 100644
--- a/src/blenderbim/blenderbim/bim/module/model/wall.py
+++ b/src/blenderbim/blenderbim/bim/module/model/wall.py
@@ -37,6 +37,7 @@ from blenderbim.bim.ifc import IfcStore
 from math import pi, sin, cos, degrees, radians
 from mathutils import Vector, Matrix
 from blenderbim.bim.module.model.opening import FilledOpeningGenerator
+from typing import Optional
 
 
 class JoinWall(bpy.types.Operator, tool.Ifc.Operator):
@@ -608,7 +609,7 @@ class DumbWallGenerator:
             should_add_representation=False,
             context=self.body_context,
         )
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=self.relating_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=self.relating_type)
         if self.axis_context:
             representation = ifcopenshell.api.run(
                 "geometry.add_axis_representation",
@@ -729,18 +730,29 @@ class DumbWallPlaner:
         DumbWallRecalculator().recalculate([w for w in set(walls) if w])
 
     def regenerate_from_type(self, usecase_path, ifc_file, settings):
-        obj = tool.Ifc.get_object(settings["related_object"])
-        if not obj or not obj.data or not obj.data.BIMMeshProperties.ifc_definition_id:
-            return
-        self.unit_scale = ifcopenshell.util.unit.calculate_unit_scale(ifc_file)
-        new_material = ifcopenshell.util.element.get_material(settings["relating_type"])
+        relating_type = settings["relating_type"]
+
+        new_material = ifcopenshell.util.element.get_material(relating_type)
         if not new_material or not new_material.is_a("IfcMaterialLayerSet"):
             return
-        parametric = ifcopenshell.util.element.get_psets(settings["relating_type"]).get("EPset_Parametric")
+
+        parametric = ifcopenshell.util.element.get_psets(relating_type).get("EPset_Parametric")
         layer_set_direction = None
         if parametric:
             layer_set_direction = parametric.get("LayerSetDirection", layer_set_direction)
-        material = ifcopenshell.util.element.get_material(settings["related_object"])
+
+        self.unit_scale = ifcopenshell.util.unit.calculate_unit_scale(ifc_file)
+        for related_object in settings["related_objects"]:
+            self._regenerate_from_type(related_object, layer_set_direction)
+
+    def _regenerate_from_type(
+        self, related_object: ifcopenshell.entity_instance, layer_set_direction: Optional[str]
+    ) -> None:
+        obj = tool.Ifc.get_object(related_object)
+        if not obj or not obj.data or not obj.data.BIMMeshProperties.ifc_definition_id:
+            return
+
+        material = ifcopenshell.util.element.get_material(related_object)
         if not material or not material.is_a("IfcMaterialLayerSetUsage"):
             return
         if layer_set_direction:
diff --git a/src/blenderbim/blenderbim/core/type.py b/src/blenderbim/blenderbim/core/type.py
index ec36ad5ffe..969dc3c125 100644
--- a/src/blenderbim/blenderbim/core/type.py
+++ b/src/blenderbim/blenderbim/core/type.py
@@ -20,7 +20,7 @@ import blenderbim.core.geometry
 
 
 def assign_type(ifc, type_tool, element=None, type=None):
-    ifc.run("type.assign_type", related_object=element, relating_type=type)
+    ifc.run("type.assign_type", related_objects=[element], relating_type=type)
     obj = ifc.get_object(element)
     if type_tool.has_material_usage(element):
         pass # for now, representation regeneration handled by API listeners
diff --git a/src/blenderbim/blenderbim/tool/spatial.py b/src/blenderbim/blenderbim/tool/spatial.py
index 4e5afa9682..48d2b54274 100644
--- a/src/blenderbim/blenderbim/tool/spatial.py
+++ b/src/blenderbim/blenderbim/tool/spatial.py
@@ -690,7 +690,7 @@ class Spatial(blenderbim.core.tool.Spatial):
         instance_class = ifcopenshell.util.type.get_applicable_entities(ifc_class, tool.Ifc.get().schema)[0]
         bpy.ops.bim.assign_class(obj=obj.name, ifc_class=instance_class)
         element = tool.Ifc.get_entity(obj)
-        tool.Ifc.run("type.assign_type", related_object=element, relating_type=relating_type)
+        tool.Ifc.run("type.assign_type", related_objects=[element], relating_type=relating_type)
 
     @classmethod
     def assign_relating_type_to_element(cls, ifc, Type, element, relating_type):
diff --git a/src/blenderbim/test/core/test_type.py b/src/blenderbim/test/core/test_type.py
index 0a084f8438..5800f04ea4 100644
--- a/src/blenderbim/test/core/test_type.py
+++ b/src/blenderbim/test/core/test_type.py
@@ -22,7 +22,7 @@ from test.core.bootstrap import ifc, type
 
 class TestAssignType:
     def test_assigning_and_switching_to_an_existing_type_data(self, ifc, type):
-        ifc.run("type.assign_type", related_object="element", relating_type="type").should_be_called()
+        ifc.run("type.assign_type", related_objects=["element"], relating_type="type").should_be_called()
         type.has_material_usage("element").should_be_called().will_return(False)
         ifc.get_object("type").should_be_called().will_return("type_obj")
         type.get_object_data("type_obj").should_be_called().will_return("type_obj_data")
@@ -32,7 +32,7 @@ class TestAssignType:
         subject.assign_type(ifc, type, element="element", type="type")
 
     def test_assigning_and_not_changing_data_if_the_type_has_no_data(self, ifc, type):
-        ifc.run("type.assign_type", related_object="element", relating_type="type").should_be_called()
+        ifc.run("type.assign_type", related_objects=["element"], relating_type="type").should_be_called()
         type.has_material_usage("element").should_be_called().will_return(False)
         ifc.get_object("type").should_be_called().will_return("type_obj")
         type.get_object_data("type_obj").should_be_called().will_return(None)
diff --git a/src/blenderbim/test/tool/test_geometry.py b/src/blenderbim/test/tool/test_geometry.py
index 48f443ad19..0837fe50a3 100644
--- a/src/blenderbim/test/tool/test_geometry.py
+++ b/src/blenderbim/test/tool/test_geometry.py
@@ -199,7 +199,7 @@ class TestGetElementType(NewFile):
         ifc = tool.Ifc.get()
         element = ifc.createIfcWall()
         type = ifc.createIfcWallType()
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=element, relating_type=type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[element], relating_type=type)
         assert subject.get_element_type(element) == type
 
 
@@ -209,7 +209,7 @@ class TestGetElementsOfType(NewFile):
         ifc = tool.Ifc.get()
         element = ifc.createIfcWall()
         type = ifc.createIfcWallType()
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=element, relating_type=type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[element], relating_type=type)
         assert subject.get_elements_of_type(type) == (element,)
 
 
diff --git a/src/blenderbim/test/tool/test_root.py b/src/blenderbim/test/tool/test_root.py
index 099880df3f..7e0e40fed4 100644
--- a/src/blenderbim/test/tool/test_root.py
+++ b/src/blenderbim/test/tool/test_root.py
@@ -98,7 +98,7 @@ class TestGetElementType(NewFile):
         ifc = tool.Ifc.get()
         element = ifc.createIfcWall()
         type = ifc.createIfcWallType()
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=element, relating_type=type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[element], relating_type=type)
         assert subject.get_element_type(element) == type
 
 
diff --git a/src/blenderbim/test/tool/test_type.py b/src/blenderbim/test/tool/test_type.py
index 4e3a7342f5..bf497e021c 100644
--- a/src/blenderbim/test/tool/test_type.py
+++ b/src/blenderbim/test/tool/test_type.py
@@ -154,7 +154,7 @@ class TestGetTypeOccurrences(NewFile):
         tool.Ifc.set(ifc)
         wall_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType")
         wall = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=wall, relating_type=wall_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[wall], relating_type=wall_type)
         assert subject.get_type_occurrences(wall_type) == (wall,)
 
 
diff --git a/src/ifcopenshell-python/docs/ifcopenshell-python/geometry_creation.rst b/src/ifcopenshell-python/docs/ifcopenshell-python/geometry_creation.rst
index f79b7c8d80..408329c078 100644
--- a/src/ifcopenshell-python/docs/ifcopenshell-python/geometry_creation.rst
+++ b/src/ifcopenshell-python/docs/ifcopenshell-python/geometry_creation.rst
@@ -648,7 +648,7 @@ efficient and implies that the type is interchangable (e.g. for maintenance).
 
     # Assign our furniture occurrence to the type.
     # That's it! The representation will automatically be mapped!
-    run("type.assign_type", model, related_object=element, relating_type=element_type)
+    run("type.assign_type", model, related_objects=[element], relating_type=element_type)
 
 Material layer sets
 -------------------
@@ -693,7 +693,7 @@ responsibility to make sure the geometry is correct.
     ifcopenshell.api.run("geometry.edit_object_placement", model, product=wall)
 
     # The wall is a WAL01 wall type. The material layer set is inherited.
-    ifcopenshell.api.run("type.assign_type", model, related_object=wall, relating_type=wall_type)
+    ifcopenshell.api.run("type.assign_type", model, related_objects=[wall], relating_type=wall_type)
 
     # It's now our responsibility to create a compatible representation.
     # Notice how our thickness of 0.118 must equal .013 + .092 + .013 from our type
@@ -749,7 +749,7 @@ responsibility to make sure the geometry is correct.
     ifcopenshell.api.run("geometry.edit_object_placement", model, product=beam)
 
     # The beam is a B1 beam type. The material profile set is inherited.
-    ifcopenshell.api.run("type.assign_type", model, related_object=beam, relating_type=beam_type)
+    ifcopenshell.api.run("type.assign_type", model, related_objects=[beam], relating_type=beam_type)
 
     # It's now our responsibility to create a compatible representation.
     # Notice how we reuse our profile instead of creating a new profile.
diff --git a/src/ifcopenshell-python/ifcopenshell/api/__init__.py b/src/ifcopenshell-python/ifcopenshell/api/__init__.py
index fea3e6a225..ccea070c98 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/__init__.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/__init__.py
@@ -73,6 +73,9 @@ ARGUMENTS_DEPRECATION = {
     "nest.unassign_object": partial(
         batching_argument_deprecation, prev_argument="related_object", new_argument="related_objects"
     ),
+    "type.assign_type": partial(
+        batching_argument_deprecation, prev_argument="related_object", new_argument="related_objects"
+    ),
 }
 
 
diff --git a/src/ifcopenshell-python/ifcopenshell/api/material/assign_material.py b/src/ifcopenshell-python/ifcopenshell/api/material/assign_material.py
index 0c27e4ea46..b1a97faaac 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/material/assign_material.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/material/assign_material.py
@@ -85,8 +85,8 @@ class Usecase:
             # automatically inherit the material from the type.
             bench1 = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcFurniture")
             bench2 = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcFurniture")
-            ifcopenshell.api.run("type.assign_type", model, related_object=bench1, relating_type=bench_type)
-            ifcopenshell.api.run("type.assign_type", model, related_object=bench2, relating_type=bench_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[bench1], relating_type=bench_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[bench2], relating_type=bench_type)
 
             # If we have a concrete wall, we should use a layer set. Again,
             # let's start with a wall type, not occurrences.
@@ -106,7 +106,7 @@ class Usecase:
 
             # Let's imagine an occurrence of this wall type.
             wall = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
-            ifcopenshell.api.run("type.assign_type", model, related_object=wall, relating_type=wall_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[wall], relating_type=wall_type)
 
             # Our wall occurrence needs to have a "set usage" which describes
             # how the layers relate to a reference line (typically a 2D line
diff --git a/src/ifcopenshell-python/ifcopenshell/api/material/edit_layer_usage.py b/src/ifcopenshell-python/ifcopenshell/api/material/edit_layer_usage.py
index d6900314ab..63aa2adda1 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/material/edit_layer_usage.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/material/edit_layer_usage.py
@@ -59,7 +59,7 @@ class Usecase:
 
             # Let's imagine an occurrence of this wall type.
             wall = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
-            ifcopenshell.api.run("type.assign_type", model, related_object=wall, relating_type=wall_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[wall], relating_type=wall_type)
 
             # Our wall occurrence needs to have a "set usage" which describes
             # how the layers relate to a reference line (typically a 2D line
diff --git a/src/ifcopenshell-python/ifcopenshell/api/project/append_asset.py b/src/ifcopenshell-python/ifcopenshell/api/project/append_asset.py
index 7ac21e07f3..991824859b 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/project/append_asset.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/project/append_asset.py
@@ -204,7 +204,7 @@ class Usecase:
                 "type.assign_type",
                 self.file,
                 should_run_listeners=False,
-                related_object=element,
+                related_objects=[element],
                 relating_type=new_type,
                 should_map_representations=False,
             )
diff --git a/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py b/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py
index 622f034b79..260b068e7e 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/type/assign_type.py
@@ -19,11 +19,18 @@
 import ifcopenshell
 import ifcopenshell.api
 import ifcopenshell.util.element
+from typing import Union
 
 
 class Usecase:
-    def __init__(self, file, related_object=None, relating_type=None, should_map_representations=True):
-        """Assigns a type to an occurrence of an object
+    def __init__(
+        self,
+        file: ifcopenshell.file,
+        related_objects: list[ifcopenshell.entity_instance],
+        relating_type: ifcopenshell.entity_instance,
+        should_map_representations=True,
+    ):
+        """Assigns a type to occurrences of an object
 
         IFC supports the concept of occurrences and types. An occurrence is an
         actual physical product in the real world: like a wall, a chair, a door,
@@ -83,8 +90,8 @@ class Usecase:
         as-built or dilapidation models, where existing conditions are
         ambiguous, unknown or are so bespoke as to have no logical type.
 
-        :param related_object: The IfcElement occurrence.
-        :type related_object: ifcopenshell.entity_instance.entity_instance
+        :param related_objects: The IfcElement occurrences.
+        :type related_objects: list[ifcopenshell.entity_instance.entity_instance]
         :param relating_type: The IfcElementType type.
         :type relating_type: ifcopenshell.entity_instance.entity_instance
         :param should_map_representations: If a type has a representation map,
@@ -93,7 +100,8 @@ class Usecase:
             yourself. In this scenario, you may set this to False.
         :type should_map_representations: bool
         :return: The IfcRelDefinesByType relationship
-        :rtype: ifcopenshell.entity_instance.entity_instance
+            or `None` if `related_objects` was empty list.
+        :rtype: Union[ifcopenshell.entity_instance.entity_instance, None]
 
         Example:
 
@@ -111,7 +119,7 @@ class Usecase:
             # had a representation, the furniture occurrence will also now have
             # the exact same representation. This is highly efficient as you
             # don't need to define the representation for every occurrence.
-            ifcopenshell.api.run("type.assign_type", model, related_object=furniture, relating_type=furniture_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[furniture], relating_type=furniture_type)
 
             # Let's imagine a parametric material layer set
             wall_type = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWallType", name="WAL01")
@@ -146,7 +154,7 @@ class Usecase:
             wall = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
 
             # The wall is a WAL01 wall type.
-            ifcopenshell.api.run("type.assign_type", model, related_object=wall, relating_type=wall_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[wall], relating_type=wall_type)
 
             # A bit of preparation, let's create some geometric contexts since
             # we want to create some geometry for our wall.
@@ -167,81 +175,101 @@ class Usecase:
         """
         self.file = file
         self.settings = {
-            "related_object": related_object,
+            "related_objects": related_objects,
             "relating_type": relating_type,
             "should_map_representations": should_map_representations,
         }
 
-    def execute(self):
-        if self.file.schema == "IFC2X3":
-            is_typed_by = None
-            is_defined_by = self.settings["related_object"].IsDefinedBy
-            for rel in is_defined_by:
-                if rel.is_a("IfcRelDefinesByType"):
-                    is_typed_by = [rel]
-                    break
-            types = self.settings["relating_type"].ObjectTypeOf
-        else:
-            is_typed_by = self.settings["related_object"].IsTypedBy
-            types = self.settings["relating_type"].Types
-
-        if types and is_typed_by == types:
+    def execute(self) -> Union[ifcopenshell.entity_instance, None]:
+        if not self.settings["related_objects"]:
             return
 
-        if is_typed_by:
-            related_objects = list(is_typed_by[0].RelatedObjects)
-            related_objects.remove(self.settings["related_object"])
-            if related_objects:
-                is_typed_by[0].RelatedObjects = related_objects
-                ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": is_typed_by[0]})
+        related_objects = set(self.settings["related_objects"])
+        relating_type = self.settings["relating_type"]
+
+        ifc2x3 = self.file.schema == "IFC2X3"
+
+        related_objects = set(self.settings["related_objects"])
+        relating_type = self.settings["relating_type"]
+        if ifc2x3:
+            types = next(iter(relating_type.ObjectTypeOf), None)
+        else:
+            types = next(iter(relating_type.Types), None)
+
+        previous_types_rels: set[ifcopenshell.entity_instance] = set()
+        objects_without_types: list[ifcopenshell.entity_instance] = []
+        objects_with_types: list[ifcopenshell.entity_instance] = []
+
+        # check if there is anything to change
+        for object in related_objects:
+            if ifc2x3:
+                object_rel = next((i for i in object.IsDefinedBy if i.is_a("IfcRelDefinesByType")), None)
             else:
-                history = is_typed_by[0].OwnerHistory
-                self.file.remove(is_typed_by[0])
+                object_rel = next(iter(object.IsTypedBy), None)
+
+            if object_rel is None:
+                objects_without_types.append(object)
+                continue
+
+            # either is_nested_by is None or product is part of different rel
+            if object_rel != types:
+                previous_types_rels.add(object_rel)
+                objects_with_types.append(object)
+
+        objects_to_change = objects_without_types + objects_with_types
+        # nothing to change
+        if not objects_to_change:
+            return types
+
+        # unassign from previous types
+        for is_typed_by in previous_types_rels:
+            cur_related_objects = set(is_typed_by.RelatedObjects) - related_objects
+            if cur_related_objects:
+                is_typed_by.RelatedObjects = list(cur_related_objects)
+                ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": is_typed_by})
+            else:
+                history = is_typed_by.OwnerHistory
+                self.file.remove(is_typed_by)
                 if history:
                     ifcopenshell.util.element.remove_deep2(self.file, history)
 
+        # assign objects to a new type
         if types:
-            related_objects = list(types[0].RelatedObjects)
-            related_objects.append(self.settings["related_object"])
-            types[0].RelatedObjects = related_objects
-            ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": types[0]})
+            types.RelatedObjects = list(set(types.RelatedObjects) | related_objects)
+            ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": types})
         else:
             types = self.file.create_entity(
                 "IfcRelDefinesByType",
                 **{
                     "GlobalId": ifcopenshell.guid.new(),
                     "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", self.file),
-                    "RelatedObjects": [self.settings["related_object"]],
-                    "RelatingType": self.settings["relating_type"],
-                }
+                    "RelatedObjects": list(related_objects),
+                    "RelatingType": relating_type,
+                },
             )
 
         if self.settings["should_map_representations"]:
-            if getattr(self.settings["relating_type"], "RepresentationMaps", None):
-                ifcopenshell.api.run(
-                    "type.map_type_representations",
-                    self.file,
-                    related_object=self.settings["related_object"],
-                    relating_type=self.settings["relating_type"],
-                )
-            self.map_material_usages()
+            if getattr(relating_type, "RepresentationMaps", None):
+                for related_object in related_objects:
+                    ifcopenshell.api.run(
+                        "type.map_type_representations",
+                        self.file,
+                        related_object=related_object,
+                        relating_type=relating_type,
+                    )
+            self.map_material_usages(related_objects)
         return types
 
-    def map_material_usages(self):
+    def map_material_usages(self, related_objects: set[ifcopenshell.entity_instance]) -> None:
         type_material = ifcopenshell.util.element.get_material(self.settings["relating_type"])
         if not type_material:
             return
-        if type_material.is_a("IfcMaterialLayerSet"):
-            ifcopenshell.api.run(
-                "material.assign_material",
-                self.file,
-                product=self.settings["related_object"],
-                type="IfcMaterialLayerSetUsage",
-            )
-        elif type_material.is_a("IfcMaterialProfileSet"):
-            ifcopenshell.api.run(
-                "material.assign_material",
-                self.file,
-                product=self.settings["related_object"],
-                type="IfcMaterialProfileSetUsage",
-            )
+        ifc_class = type_material.is_a()
+        if ifc_class in ("IfcMaterialLayerSet", "IfcMaterialProfileSet"):
+            for related_object in related_objects:
+                ifcopenshell.api.run(
+                    "material.assign_material",
+                    self.file,
+                    product=related_object,
+                    type=f"{ifc_class}Usage",
+                )
diff --git a/src/ifcopenshell-python/ifcopenshell/api/type/map_type_representations.py b/src/ifcopenshell-python/ifcopenshell/api/type/map_type_representations.py
index 940fd25902..180e163477 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/type/map_type_representations.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/type/map_type_representations.py
@@ -60,7 +60,7 @@ class Usecase:
             # furniture type has no representation, so the furniture may also
             # have no representation, or any arbitrary representation that may
             # vary from occurrence to occurrence.
-            ifcopenshell.api.run("type.assign_type", model, related_object=furniture, relating_type=furniture_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[furniture], relating_type=furniture_type)
 
             # A bit of preparation, let's create some geometric contexts since
             # we want to create some geometry for our furniture type.
diff --git a/src/ifcopenshell-python/ifcopenshell/api/type/unassign_type.py b/src/ifcopenshell-python/ifcopenshell/api/type/unassign_type.py
index 843121f967..dbafade861 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/type/unassign_type.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/type/unassign_type.py
@@ -42,7 +42,7 @@ class Usecase:
             furniture = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcFurniture")
 
             # Assign the furniture to the furniture type.
-            ifcopenshell.api.run("type.assign_type", model, related_object=furniture, relating_type=furniture_type)
+            ifcopenshell.api.run("type.assign_type", model, related_objects=[furniture], relating_type=furniture_type)
 
             # Change our mind. Maybe it's a different type?
             ifcopenshell.api.run("type.unassign_type", model, related_object=furniture)
diff --git a/src/ifcopenshell-python/test/api/geometry/test_assign_representation.py b/src/ifcopenshell-python/test/api/geometry/test_assign_representation.py
index 1bdd7f1ce8..654e4d383a 100644
--- a/src/ifcopenshell-python/test/api/geometry/test_assign_representation.py
+++ b/src/ifcopenshell-python/test/api/geometry/test_assign_representation.py
@@ -47,7 +47,7 @@ class TestAssignRepresentation(test.bootstrap.IFC4):
     def test_assigning_to_a_type_will_map_representations_to_instances(self):
         wall = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         walltype = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=wall, relating_type=walltype)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[wall], relating_type=walltype)
         context = self.file.createIfcGeometricRepresentationContext()
         rep = self.file.createIfcShapeRepresentation(ContextOfItems=context)
         rep2 = self.file.createIfcShapeRepresentation(ContextOfItems=context)
@@ -69,7 +69,7 @@ class TestAssignRepresentation(test.bootstrap.IFC4):
         ifcopenshell.api.run("geometry.assign_representation", self.file, product=walltype, representation=rep)
 
         wall = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=wall, relating_type=walltype)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[wall], relating_type=walltype)
 
         ifcopenshell.api.run("geometry.assign_representation", self.file, product=wall, representation=rep2)
         assert wall.Representation.Representations[0].RepresentationType == "MappedRepresentation"
@@ -84,7 +84,7 @@ class TestAssignRepresentation(test.bootstrap.IFC4):
         rep = self.file.createIfcShapeRepresentation(ContextOfItems=context)
         walltype = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         wall = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=wall, relating_type=walltype)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[wall], relating_type=walltype)
         ifcopenshell.api.run("geometry.assign_representation", self.file, product=wall, representation=rep)
         assert wall.Representation.Representations[0].RepresentationType != "MappedRepresentation"
         assert wall.Representation.Representations[0] == rep
diff --git a/src/ifcopenshell-python/test/api/material/test_assign_material.py b/src/ifcopenshell-python/test/api/material/test_assign_material.py
index 9e616cbb95..dabcdb811c 100644
--- a/src/ifcopenshell-python/test/api/material/test_assign_material.py
+++ b/src/ifcopenshell-python/test/api/material/test_assign_material.py
@@ -48,7 +48,7 @@ class TestAssignMaterial(test.bootstrap.IFC4):
     def test_assign_type_material_layer_set_and_element_layer_set_usage(self):
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file, name="CON01")
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, type="IfcMaterialLayerSet")
         ifcopenshell.api.run("material.assign_material", self.file, product=element, type="IfcMaterialLayerSetUsage")
@@ -69,7 +69,7 @@ class TestAssignMaterial(test.bootstrap.IFC4):
     def test_assign_type_material_profile_set_and_element_profile_set_usage(self):
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file, name="CON01")
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, type="IfcMaterialProfileSet")
         ifcopenshell.api.run("material.assign_material", self.file, product=element, type="IfcMaterialProfileSetUsage")
diff --git a/src/ifcopenshell-python/test/api/material/test_unassign_material.py b/src/ifcopenshell-python/test/api/material/test_unassign_material.py
index aea9e78ddf..9634941d21 100644
--- a/src/ifcopenshell-python/test/api/material/test_unassign_material.py
+++ b/src/ifcopenshell-python/test/api/material/test_unassign_material.py
@@ -62,7 +62,7 @@ class TestUnassignMaterial(test.bootstrap.IFC4):
     def test_unassign_material_layer_set_usage_from_element(self):
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file, name="CON01")
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, type="IfcMaterialLayerSet")
         ifcopenshell.api.run("material.assign_material", self.file, product=element, type="IfcMaterialLayerSetUsage")
@@ -78,8 +78,8 @@ class TestUnassignMaterial(test.bootstrap.IFC4):
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element2, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element2], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file, name="CON01")
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, type="IfcMaterialLayerSet")
         rel = ifcopenshell.api.run(
@@ -114,7 +114,7 @@ class TestUnassignMaterial(test.bootstrap.IFC4):
     def test_unassign_material_profile_set_usage_from_element(self):
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file, name="CON01")
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, type="IfcMaterialProfileSet")
         ifcopenshell.api.run("material.assign_material", self.file, product=element, type="IfcMaterialProfileSetUsage")
diff --git a/src/ifcopenshell-python/test/api/project/test_append_asset.py b/src/ifcopenshell-python/test/api/project/test_append_asset.py
index bca7a1580d..7c8ed8943a 100644
--- a/src/ifcopenshell-python/test/api/project/test_append_asset.py
+++ b/src/ifcopenshell-python/test/api/project/test_append_asset.py
@@ -88,7 +88,7 @@ class TestAppendAsset(test.bootstrap.IFC4):
         element = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
         element_type2 = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", library, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", library, name="Material")
         ifcopenshell.api.run("material.assign_material", library, product=element, material=material)
         ifcopenshell.api.run("material.assign_material", library, product=element_type, material=material)
@@ -186,7 +186,7 @@ class TestAppendAsset(test.bootstrap.IFC4):
         element_type.RepresentationMaps = [library.createIfcRepresentationMap(MappedRepresentation=mapped_rep)]
 
         element = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", library, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element], relating_type=element_type)
 
         ifcopenshell.api.run("project.append_asset", self.file, library=library, element=element)
         assert len(self.file.by_type("IfcStyledItem")) == 1
@@ -356,7 +356,7 @@ class TestAppendAsset(test.bootstrap.IFC4):
         library = ifcopenshell.api.run("project.create_file")
         element = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", library, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element], relating_type=element_type)
         ifcopenshell.api.run("project.append_asset", self.file, library=library, element=element)
         assert ifcopenshell.util.element.get_type(self.file.by_type("IfcWall")[0]).is_a("IfcWallType")
 
@@ -366,9 +366,9 @@ class TestAppendAsset(test.bootstrap.IFC4):
         element2 = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element3 = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", library, related_object=element, relating_type=element_type)
-        ifcopenshell.api.run("type.assign_type", library, related_object=element2, relating_type=element_type)
-        ifcopenshell.api.run("type.assign_type", library, related_object=element3, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element], relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element2], relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element3], relating_type=element_type)
         ifcopenshell.api.run("project.append_asset", self.file, library=library, element=element)
         ifcopenshell.api.run("project.append_asset", self.file, library=library, element=element2)
         assert len(ifcopenshell.util.element.get_types(self.file.by_type("IfcWallType")[0])) == 2
@@ -387,7 +387,7 @@ class TestAppendAsset(test.bootstrap.IFC4):
         element = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
         element_type2 = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", library, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", library, name="Material")
         ifcopenshell.api.run("material.assign_material", library, product=element, material=material)
         ifcopenshell.api.run("material.assign_material", library, product=element_type, material=material)
@@ -419,7 +419,7 @@ class TestAppendAssetIFC2X3(test.bootstrap.IFC2X3):
         library = ifcopenshell.api.run("project.create_file", version="IFC2X3")
         element = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", library, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element], relating_type=element_type)
         ifcopenshell.api.run("project.append_asset", self.file, library=library, element=element)
         assert ifcopenshell.util.element.get_type(self.file.by_type("IfcWall")[0]).is_a("IfcWallType")
 
@@ -429,9 +429,9 @@ class TestAppendAssetIFC2X3(test.bootstrap.IFC2X3):
         element2 = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element3 = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", library, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", library, related_object=element, relating_type=element_type)
-        ifcopenshell.api.run("type.assign_type", library, related_object=element2, relating_type=element_type)
-        ifcopenshell.api.run("type.assign_type", library, related_object=element3, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element], relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element2], relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", library, related_objects=[element3], relating_type=element_type)
         ifcopenshell.api.run("project.append_asset", self.file, library=library, element=element)
         ifcopenshell.api.run("project.append_asset", self.file, library=library, element=element2)
         assert len(ifcopenshell.util.element.get_types(self.file.by_type("IfcWallType")[0])) == 2
diff --git a/src/ifcopenshell-python/test/api/root/test_copy_class.py b/src/ifcopenshell-python/test/api/root/test_copy_class.py
index a4177150fa..25e2b06345 100644
--- a/src/ifcopenshell-python/test/api/root/test_copy_class.py
+++ b/src/ifcopenshell-python/test/api/root/test_copy_class.py
@@ -210,7 +210,7 @@ class TestCopyClass(test.bootstrap.IFC4):
     def test_copying_a_type_and_purging_type_relationships(self):
         type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=type)
         new = ifcopenshell.api.run("root.copy_class", self.file, product=type)
         assert not new.Types
 
diff --git a/src/ifcopenshell-python/test/api/root/test_reassign_class.py b/src/ifcopenshell-python/test/api/root/test_reassign_class.py
index fc9a7f4b2b..17b3fd1801 100644
--- a/src/ifcopenshell-python/test/api/root/test_reassign_class.py
+++ b/src/ifcopenshell-python/test/api/root/test_reassign_class.py
@@ -54,9 +54,9 @@ class TestReassignClass(test.bootstrap.IFC4):
     def test_reassign_class_for_type_occurrences(self):
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element1, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element1], relating_type=element_type)
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element2, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element2], relating_type=element_type)
 
         element_type = ifcopenshell.api.run(
             "root.reassign_class", self.file, product=element_type, ifc_class="IfcSlabType"
@@ -74,9 +74,9 @@ class TestReassignClass(test.bootstrap.IFC4):
     def test_reassigning_type_class_and_its_occurrences_classes_if_entity_was_typed(self):
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element1, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element1], relating_type=element_type)
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element2, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element2], relating_type=element_type)
 
         element1 = ifcopenshell.api.run("root.reassign_class", self.file, product=element1, ifc_class="IfcSlab")
 
diff --git a/src/ifcopenshell-python/test/api/root/test_remove_product.py b/src/ifcopenshell-python/test/api/root/test_remove_product.py
index 34756288bd..821e9cb3dc 100644
--- a/src/ifcopenshell-python/test/api/root/test_remove_product.py
+++ b/src/ifcopenshell-python/test/api/root/test_remove_product.py
@@ -98,7 +98,7 @@ class TestRemoveProduct(test.bootstrap.IFC4):
         context = ifcopenshell.api.run("context.add_context", self.file, context_type="Model")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         rep_map = self.file.createIfcRepresentationMap(
             MappingOrigin=self.file.createIfcAxis2Placement3D(),
             MappedRepresentation=self.file.createIfcShapeRepresentation(
@@ -377,7 +377,7 @@ class TestRemoveProduct(test.bootstrap.IFC4):
     def test_removing_all_type_relationships_of_an_element(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         ifcopenshell.api.run("root.remove_product", self.file, product=element)
         assert not self.file.by_type("IfcRelDefinesByType")
         assert self.file.by_type("IfcWallType")
@@ -386,8 +386,8 @@ class TestRemoveProduct(test.bootstrap.IFC4):
         element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element1, relating_type=element_type)
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element2, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element1], relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element2], relating_type=element_type)
         ifcopenshell.api.run("root.remove_product", self.file, product=element_type)
         assert not self.file.by_type("IfcRelDefinesByType")
         assert self.file.by_type("IfcWall")
diff --git a/src/ifcopenshell-python/test/api/test_api.py b/src/ifcopenshell-python/test/api/test_api.py
index 15e81b03ba..c19c378427 100644
--- a/src/ifcopenshell-python/test/api/test_api.py
+++ b/src/ifcopenshell-python/test/api/test_api.py
@@ -123,3 +123,11 @@ class TestTemporarySupportForDeprecatedAPIArguments(test.bootstrap.IFC4):
         ifcopenshell.api.run("nest.assign_object", self.file, related_objects=[subelement1], relating_object=element)
         ifcopenshell.api.run("nest.unassign_object", self.file, related_object=subelement1)
         assert ifcopenshell.util.element.get_nest(subelement1) is None
+
+    @deprecation_check
+    def test_assigning_a_type(self):
+        element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
+        element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        rel = ifcopenshell.api.run("type.assign_type", self.file, related_object=element1, relating_type=element_type)
+        assert ifcopenshell.util.element.get_type(element1) == element_type
+        assert rel.is_a("IfcRelDefinesByType")
diff --git a/src/ifcopenshell-python/test/api/type/test_assign_type.py b/src/ifcopenshell-python/test/api/type/test_assign_type.py
new file mode 100644
index 0000000000..484cceb260
--- /dev/null
+++ b/src/ifcopenshell-python/test/api/type/test_assign_type.py
@@ -0,0 +1,70 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2021 Dion Moult <dion@thinkmoult.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+import test.bootstrap
+import ifcopenshell.api
+import ifcopenshell.util.element
+import pytest
+
+
+class TestAssignType(test.bootstrap.IFC4):
+    def test_assigning_a_type(self):
+        element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
+        element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
+        element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        rel = ifcopenshell.api.run(
+            "type.assign_type", self.file, related_objects=[element1, element2], relating_type=element_type
+        )
+        assert ifcopenshell.util.element.get_type(element1) == element_type
+        assert ifcopenshell.util.element.get_type(element2) == element_type
+        assert rel.is_a("IfcRelDefinesByType")
+
+    def test_doing_nothing_if_type_is_already_assigned(self):
+        element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
+        element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
+        total_elements = len([e for e in self.file])
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
+        assert len([e for e in self.file]) == total_elements
+
+    def test_that_old_typing_relationships_are_updated_if_they_still_have_elements(self):
+        element_type1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element_type2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
+        element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
+        ifcopenshell.api.run(
+            "type.assign_type", self.file, related_objects=[element1, element2], relating_type=element_type1
+        )
+        rel = element1.IsDefinedBy[0] if self.file.schema == "IFC2X3" else element1.IsTypedBy[0]
+        assert len(rel.RelatedObjects) == 2
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element1], relating_type=element_type2)
+        assert len(rel.RelatedObjects) == 1
+
+    def test_that_old_typing_relationships_are_purged_if_no_more_elements_are_nested(self):
+        element_type1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element_type2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element1], relating_type=element_type1)
+        rel_id = (element1.IsDefinedBy[0] if self.file.schema == "IFC2X3" else element1.IsTypedBy[0]).id()
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element1], relating_type=element_type2)
+        with pytest.raises(RuntimeError):
+            self.file.by_id(rel_id)
+
+
+class TestAssignTypeIFC2X3(test.bootstrap.IFC2X3, TestAssignType):
+    pass
diff --git a/src/ifcopenshell-python/test/api/type/test_map_type_representation.py b/src/ifcopenshell-python/test/api/type/test_map_type_representation.py
index 50e63cb90d..a43430a040 100644
--- a/src/ifcopenshell-python/test/api/type/test_map_type_representation.py
+++ b/src/ifcopenshell-python/test/api/type/test_map_type_representation.py
@@ -24,7 +24,7 @@ class TestMapTypeRepresentations(test.bootstrap.IFC4):
     def test_doing_nothing_if_the_type_has_no_representation_maps(self):
         element = self.file.createIfcWall()
         type = self.file.createIfcWallType()
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=type)
         total_elements = len([e for e in self.file])
         ifcopenshell.api.run("type.map_type_representations", self.file, related_object=element, relating_type=type)
         assert len([e for e in self.file]) == total_elements
diff --git a/src/ifcopenshell-python/test/util/test_brick.py b/src/ifcopenshell-python/test/util/test_brick.py
index 52a99b1a1a..78b7771d57 100644
--- a/src/ifcopenshell-python/test/util/test_brick.py
+++ b/src/ifcopenshell-python/test/util/test_brick.py
@@ -36,5 +36,5 @@ class TestGetBrickTypeIFC2X3(test.bootstrap.IFC2X3):
     def test_run(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcFlowController")
         type_element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcAirTerminalBoxType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=type_element)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=type_element)
         assert subject.get_brick_type(element) == "https://brickschema.org/schema/Brick#TerminalUnit"
diff --git a/src/ifcopenshell-python/test/util/test_classification.py b/src/ifcopenshell-python/test/util/test_classification.py
index 58a5e47de2..d394f8056b 100644
--- a/src/ifcopenshell-python/test/util/test_classification.py
+++ b/src/ifcopenshell-python/test/util/test_classification.py
@@ -69,7 +69,7 @@ class TestGetReferences(test.bootstrap.IFC4):
         project = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         ifcopenshell.api.run("classification.add_classification", self.file, classification=classification)
         ifcopenshell.api.run(
             "classification.add_reference",
@@ -98,7 +98,7 @@ class TestGetReferences(test.bootstrap.IFC4):
         project = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject")
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         ifcopenshell.api.run("classification.add_classification", self.file, classification=classification)
         ifcopenshell.api.run(
             "classification.add_reference",
diff --git a/src/ifcopenshell-python/test/util/test_element.py b/src/ifcopenshell-python/test/util/test_element.py
index ec5e686d60..1af3ac2434 100644
--- a/src/ifcopenshell-python/test/util/test_element.py
+++ b/src/ifcopenshell-python/test/util/test_element.py
@@ -55,7 +55,7 @@ class TestGetPsetIFC4(test.bootstrap.IFC4):
     def test_getting_inherited_psets(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         type_element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=type_element)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=type_element)
         pset = ifcopenshell.api.run("pset.add_pset", self.file, product=type_element, name="name")
         ifcopenshell.api.run("pset.edit_pset", self.file, pset=pset, properties={"a": 1, "x": 1})
         pset = ifcopenshell.api.run("pset.add_pset", self.file, product=element, name="name")
@@ -79,7 +79,7 @@ class TestGetPsetIFC4(test.bootstrap.IFC4):
     def test_excluding_inherited_psets(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         type_element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=type_element)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=type_element)
         pset = ifcopenshell.api.run("pset.add_pset", self.file, product=type_element, name="name")
         ifcopenshell.api.run("pset.edit_pset", self.file, pset=pset, properties={"a": 1, "x": 1})
         pset = ifcopenshell.api.run("pset.add_pset", self.file, product=element, name="name")
@@ -133,7 +133,7 @@ class TestGetPsetsIFC4(test.bootstrap.IFC4):
     def test_getting_inherited_psets(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         type_element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=type_element)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=type_element)
         pset = ifcopenshell.api.run("pset.add_pset", self.file, product=type_element, name="name")
         ifcopenshell.api.run("pset.edit_pset", self.file, pset=pset, properties={"a": 1, "x": 1})
         pset = ifcopenshell.api.run("pset.add_pset", self.file, product=element, name="name")
@@ -263,14 +263,14 @@ class TestGetPredefinedTypeIFC4(test.bootstrap.IFC4):
     def test_getting_an_inherited_predefined_type(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         element_type.PredefinedType = "PARTITIONING"
         assert subject.get_predefined_type(element) == "PARTITIONING"
 
     def test_getting_an_inherited_userdefined_type_for_an_element_type(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         element_type.PredefinedType = "USERDEFINED"
         element_type.ElementType = "FOOBAR"
         assert subject.get_predefined_type(element) == "FOOBAR"
@@ -278,7 +278,7 @@ class TestGetPredefinedTypeIFC4(test.bootstrap.IFC4):
     def test_getting_an_overriden_predefined_type(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         element_type.PredefinedType = "NOTDEFINED"
         element.PredefinedType = "PARTITIONING"
         assert subject.get_predefined_type(element) == "PARTITIONING"
@@ -286,7 +286,7 @@ class TestGetPredefinedTypeIFC4(test.bootstrap.IFC4):
     def test_getting_an_inherited_userdefined_type_for_a_process_type(self):
         element = ifcopenshell.api.run("sequence.add_task", self.file)
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcTaskType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         element_type.PredefinedType = "USERDEFINED"
         element_type.ProcessType = "FOOBAR"
         assert subject.get_predefined_type(element) == "FOOBAR"
@@ -296,7 +296,7 @@ class TestGetTypeIFC4(test.bootstrap.IFC4):
     def test_getting_the_type_of_a_product(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         assert subject.get_type(element) == element_type
         assert subject.get_type(element_type) == element_type
 
@@ -309,7 +309,7 @@ class TestGetTypes(test.bootstrap.IFC4):
     def test_getting_the_type_of_a_product(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         assert subject.get_types(element_type) == (element,)
 
 
@@ -391,7 +391,7 @@ class TestGetMaterial(test.bootstrap.IFC4):
     def test_getting_an_inherited_material_from_the_elements_type(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file)
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, material=material)
         assert subject.get_material(element) == material
@@ -399,7 +399,7 @@ class TestGetMaterial(test.bootstrap.IFC4):
     def test_getting_an_overridden_material_from_the_elements_occurrence(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file)
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, material=material)
         material = ifcopenshell.api.run("material.add_material", self.file)
@@ -409,7 +409,7 @@ class TestGetMaterial(test.bootstrap.IFC4):
     def test_getting_direct_materials_without_checking_inheritance(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file)
         ifcopenshell.api.run("material.assign_material", self.file, product=element_type, material=material)
         assert subject.get_material(element, should_inherit=False) is None
@@ -493,7 +493,7 @@ class TestGetElementsByMaterial(test.bootstrap.IFC4):
     def test_getting_elements_of_a_material_layer_set(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file)
         material_set = ifcopenshell.api.run("material.add_material_set", self.file, set_type="IfcMaterialLayerSet")
         ifcopenshell.api.run("material.add_layer", self.file, layer_set=material_set, material=material)
@@ -507,7 +507,7 @@ class TestGetElementsByMaterial(test.bootstrap.IFC4):
     def test_getting_elements_of_a_material_profile_set(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", self.file)
         material_set = ifcopenshell.api.run("material.add_material_set", self.file, set_type="IfcMaterialProfileSet")
         ifcopenshell.api.run("material.add_profile", self.file, profile_set=material_set, material=material)
diff --git a/src/ifcopenshell-python/test/util/test_selector.py b/src/ifcopenshell-python/test/util/test_selector.py
index 703558969c..6565142429 100644
--- a/src/ifcopenshell-python/test/util/test_selector.py
+++ b/src/ifcopenshell-python/test/util/test_selector.py
@@ -168,7 +168,7 @@ class TestFilterElements(test.bootstrap.IFC4):
     def test_selecting_by_type(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         assert subject.filter_elements(self.file, "IfcWall, type=Foo") == set()
         element_type.Name = "Foo"
         assert subject.filter_elements(self.file, "IfcWall, type=Foo") == {element}
@@ -465,10 +465,10 @@ class TestSelector(test.bootstrap.IFC4):
     def test_getting_occurrences_of_a_filtered_type(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element2, relating_type=element_type2)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element2], relating_type=element_type2)
         assert set(subject.Selector.parse(self.file, "* .IfcWallType")) == {element, element2}
 
     def test_getting_decomposition_of_a_filtered_type(self):
@@ -505,7 +505,7 @@ class TestSelector(test.bootstrap.IFC4):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
         element_type.Name = "Foo"
-        ifcopenshell.api.run("type.assign_type", self.file, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", self.file, related_objects=[element], relating_type=element_type)
         assert set(subject.Selector.parse(self.file, '.IfcWall[type.Name="Foo"]')) == {element}
 
     def test_selecting_via_a_material(self):
diff --git a/src/ifctester/test/ids_doc_generator.py b/src/ifctester/test/ids_doc_generator.py
index 3290104cfa..2543daec23 100644
--- a/src/ifctester/test/ids_doc_generator.py
+++ b/src/ifctester/test/ids_doc_generator.py
@@ -328,7 +328,7 @@ wall_types = []
 for i in range(0, 4):
     wall_type = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWallType", name=f"DEMO{i + 1}")
     wall = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall", name=f"WALL {i + 1}")
-    ifcopenshell.api.run("type.assign_type", model, related_object=wall, relating_type=wall_type)
+    ifcopenshell.api.run("type.assign_type", model, related_objects=[wall], relating_type=wall_type)
     representation = ifcopenshell.api.run(
         "geometry.add_wall_representation", model, context=body, length=5, height=3, thickness=(i + 1) * 0.05
     )
diff --git a/src/ifctester/test/test_facet.py b/src/ifctester/test/test_facet.py
index 65c6cfc8d2..d34e9426f7 100644
--- a/src/ifctester/test/test_facet.py
+++ b/src/ifctester/test/test_facet.py
@@ -168,7 +168,7 @@ class TestEntity:
         ifc = ifcopenshell.file()
         wall = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         wall_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType", predefined_type="X")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=wall, relating_type=wall_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[wall], relating_type=wall_type)
         facet = Entity(name="IFCWALL", predefinedType="X")
         run("Inherited predefined types should pass", facet=facet, inst=wall, expected=True)
 
@@ -177,7 +177,7 @@ class TestEntity:
         wall_type = ifcopenshell.api.run(
             "root.create_entity", ifc, ifc_class="IfcWallType", predefined_type="NOTDEFINED"
         )
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=wall, relating_type=wall_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[wall], relating_type=wall_type)
         facet = Entity(name="IFCWALL", predefinedType="X")
         run("Overridden predefined types should pass", facet=facet, inst=wall, expected=True)
 
@@ -637,7 +637,7 @@ class TestAttribute:
         ifc = ifcopenshell.file()
         wall = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         wall_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=wall, relating_type=wall_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[wall], relating_type=wall_type)
         wall_type.Description = "Foobar"
         facet = Attribute(name="Description", value="Foobar")
         run("Attributes are not inherited by the occurrence", facet=facet, inst=wall, expected=False)
@@ -808,7 +808,7 @@ class TestClassification:
         # https://github.com/buildingSMART/IFC4.3.x-development/issues/475
         wall = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         wall_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=wall, relating_type=wall_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[wall], relating_type=wall_type)
         ifcopenshell.api.run(
             "classification.add_reference", ifc, product=wall, reference=ref11, classification=system_a
         )
@@ -1209,7 +1209,7 @@ class TestProperty:
         ifc = self.setup_ifc()
         wall = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         wall_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=wall, relating_type=wall_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[wall], relating_type=wall_type)
         pset = ifcopenshell.api.run("pset.add_pset", ifc, product=wall_type, name="Foo_Bar")
         ifcopenshell.api.run("pset.edit_pset", ifc, pset=pset, properties={"Foo": "Bar"})
         facet = Property(propertySet="Foo_Bar", baseName="Foo", dataType="IFCLABEL")
@@ -1219,7 +1219,7 @@ class TestProperty:
         ifc = self.setup_ifc()
         wall = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         wall_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=wall, relating_type=wall_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[wall], relating_type=wall_type)
         pset = ifcopenshell.api.run("pset.add_pset", ifc, product=wall_type, name="Foo_Bar")
         ifcopenshell.api.run("pset.edit_pset", ifc, pset=pset, properties={"Foo": "Baz"})
         pset = ifcopenshell.api.run("pset.add_pset", ifc, product=wall, name="Foo_Bar")
@@ -1374,7 +1374,7 @@ class TestMaterial:
         ifc = ifcopenshell.file()
         element = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", ifc)
         ifcopenshell.api.run("material.assign_material", ifc, product=element_type, material=material)
         material.Name = "Foo"
@@ -1384,7 +1384,7 @@ class TestMaterial:
         ifc = ifcopenshell.file()
         element = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         element_type = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWallType")
-        ifcopenshell.api.run("type.assign_type", ifc, related_object=element, relating_type=element_type)
+        ifcopenshell.api.run("type.assign_type", ifc, related_objects=[element], relating_type=element_type)
         material = ifcopenshell.api.run("material.add_material", ifc)
         ifcopenshell.api.run("material.assign_material", ifc, product=element_type, material=material)
         material.Name = "Bar"