Virtual / external components
e.g. servos that take up space in the design, and may provide a variety of interfaces for other real component connections. They should not show up in the 2D view, but may be useful to display (with some visual distinction) in the 3D view (would need to move to a different 3d file format, e.g. OBJ instead of STL). The interfaces should be handled appropriately for faces though, e.g. paperbot wheels ending up where they belong.