The ArchiCAD Roof and slab are exported as IFCSLAB with “PredefinedType” attribute, which is “FLOOR” in case of ArchiCAD slab, and “ROOF” in case of ArchiCAD roof by default. The user can change this attribute for other IfcSlabTypeEnum. If user set it to USERDEFINED, then ObjectType attribute must be set with any text. You can type her “wall”, but it will not be interpreted as WALL element in other applications, as far as I know, because the ENTITY name will be IFCSLAB (not IFCWALL).
In case of ArchiCAD building element (wall, beam, column, slab, roof, zone, door, window, CW ..) there is no way to redefine the exported IFC entity to other IFC entity. If you use ArchiCAD object then you can redefine the exported IFC ENTITY by selecting other library subtype of the GDL library part.
