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IfcPolygonalBoundedHalfSpace PolygonalBoundary attribute

I am trying to understand the motivation behind a formal proposition about PolygonalBoundary attribute in IfcHalfSpaceSolid, IfcPolygonalBoundedHalfSpace class. Could anyone clarify why isn’t it possible to specify IfcIndexedPolyCurve as an argument?

WHERE
  BoundaryDim : PolygonalBoundary.Dim = 2;
  BoundaryType : SIZEOF(TYPEOF(PolygonalBoundary) * [
'IFCGEOMETRYRESOURCE.[IfcPolyline](https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2_TC1/HTML/schema/ifcgeometryresource/lexical/ifcpolyline.htm)',
'IFCGEOMETRYRESOURCE.[IfcCompositeCurve](https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2_TC1/HTML/schema/ifcgeometryresource/lexical/ifccompositecurve.htm)']
) = 1;

https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2_TC1/HTML/link/ifcpolygonalboundedhalfspace.htm