swarren, I agree with your comment "...I still don't know how the software determines which side of an interior doorway is the interior and which the exterior. Maybe it should be "InteriorA" and "InteriorB"? I think this is just a reuse of code for both interior doors and exterior doors." In most circumstances an interior door has both sides the same (cased or not cased). On the rare occasion where I have a doorway that only has one side cased there is no way of determining how the software decides which side of an interior door is the 'Interior / Exterior' - I just have to try and hope I pick the right side .
I may be misreading your question/problem, sorry if this is not what you are after. You can select/change each side of the doors material and trim regardless of the Interior Door type (doorway, hinged, pocket etc). To do this go to "Interior Door Specification - General". You need to tick the check box next to "Separate Trim and Materials on Each Side". You should now be able to independently change the Interior/Exterior Casing details.