Another way to think about it. All commercial CAD systems use definitions of the boundaries, the surfaces.
If you took the cross section of a CAD cube, and took any point inside the cube, there is no gemetrical information written for that specific point. The system only assesses if that point lies inside or outside the calculated solid.
If it was a real world solid, that point inside a wooden cube, would be wood. In a true “solid modeler” then each unit of space would have a definition. That file would be huge depending on resolution. Computations would not be fast.
Voxel modeling is the closest analogy. Scan and Solve uses this approach for FEA analysis.