Wanted more clear specification for IfcRepresentationContext.ContextType

"The supported values for context type are to be specified by implementers agreements." in https://standards.buildingsmart.org/IFC/RELEASE/IFC4_3/HTML/lexical/IfcRepresentationContext.htm

This gives no hint what is this agreement and where to find it.
Also we discussed many time we should go away from implementers agreements - having them means specifications are wrong and not exact.

I'm suggesting specifying base list in the schema documentation and allow MVDs to modify the list.

Start by reviewing the IfcRepresentationContext.ContextType specification at the linked IFC4.3 lexical page and the repository's schema documentation. Determine where implementer agreements are documented and how MVD-specific constraints are represented. Done means the specification gives an explicit base list and clearly explains how MVDs may modify it.

Written by the indexing model from the issue text.