I got some clarification from the developers:
An IFC translator has a “Type Mapping” preset, where a classification-based or element-based rule set can be defined. In the template shipped with AC, the classification system “ARCHICAD Classification” is set up to work with the IFC translators, always defining an IFC type and Predefined Type. Using these translators, IFC_predefinedtype has no effect.
When an IFC translator is set up to use element mapping, the mapping is not customizable by the user, and depends on the AC version. For AC23 this document shows two MEP element types where IFC_predefinedtype is handled by the translator. On the AC side, these parameters are handled by the MEP add-on.
So IFC_predefinedtype is retired, it exists in GDL elements only for compatibility.
Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE