diff --git a/nix/build-all.py b/nix/build-all.py
index 381a33770e..402630126d 100644
--- a/nix/build-all.py
+++ b/nix/build-all.py
@@ -43,9 +43,12 @@ Used environment variables:
     - ``BUILD_CFG`` - build configuration, 'RelWithDebInfo' by default.
     - ``USE_CURRENT_PYTHON_VERSION`` - use current python config instead of compile from source
     - ``IFCOS_NUM_BUILD_PROCS`` - number of concurrent processes defaults to available cores + 1
-    - ``NO_CLEAN`` - do not clean build directories but continue working on current build (installed dependencies are never cleared)
-    - ``IFCOS_SCHEMAS`` - schemas to be built; defaults to cmake default (IFC2X3; IFC4; IFC4X3_ADD2) - to be supplied as 2x3;4
-
+    - ``NO_CLEAN`` - do not clean `ifcopenshell` build directories but continue working on current build
+    (installed dependencies are never cleared).
+    By default option is disabled, to enable pass any value from `1`, `on`, `true`.
+    - ``IFCOS_SCHEMAS`` - schemas to be built; defaults to cmake default (IFC2X3; IFC4; IFC4X3_ADD2) - to be supplied as `2x3;4`
+    - ``USE_OCCT`` - whether to use official Open CASCADE instead of Community Edition
+    (`true` by default, any other value is considered `false`)
 
 # This script builds IfcOpenShell and its dependencies                        #
 #                                                                             #