If an imorted CAD file contains the appropriate surfaces, TPC imports them into the current survey as TPC surfaces.
The vertices defined by the surface are imported as survey points using DTM1, DTM2, etc as the point labels. TPC checks imported vertices to see if they match an existing survey point. If so, the existing survey point is used as the vertex. This allows you to share CAD files back and forth.
TPC imports PolyFaceMesh surfaces from AutoCAD files. Each PolyFaceMesh in the imported file creates a new surface in TPC.
The PolyFaceMesh completely defines the TIN of the surface, including all points (vertices) and faces.
TPC imports MeshFaceLoops from Bentley files. Each MeshFaceLoop in the imported file creates a new surface in TPC.
Imported surface create a traverse to hold the points associated with the surface. This traverse then becomes the source for the surface. Having unique sources is what allows TPC to compute the volume between surfaces and control the extents of a surface among other things.
The imported surface is locked so that the surface does not re-compute the TIN. You can unlock a surface and recompute the TIN if needed, but locking the surface when it is imported insures that it will be replicated in TPC the same way it was in the CAD file.