A simplified representation of the structure - beams and columns reduced to centerlines, walls reduced to planes - that structural analysis software actually calculates against, running alongside the full Physical Model (the real geometry everyone else sees and coordinates with) inside the same Revit structural elements.

The two models can drift apart if not managed carefully: moving a column's physical geometry does not always automatically move its analytical line the same way, and a structural engineer's calculations are only as trustworthy as how well the analytical model actually matches the real, physical design intent.