What you are talking about is the adjacency of a surface in AutoCAD MEP. The adjacency is actually determined by Trane Trace upon import of a gbXML file. Using AutoCAD MEP terminology, if a Wall has a Space on either side, it will be deemed Interior; if a Wall has a Space on only one side, then it is deemed Exterior. That's the way the program works.
HTH,
Kyle B
Revit MEP Product Manager (and recovering AutoCAD MEP guru)
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Kyle Bernhardt
Director
Building Design Strategy
Autodesk, Inc.