In the CAD-System, a geometry of the part is generated.
This can be done in different ways:
One way is to use simple 3D-volumes (cylinder, cuboids, etc.) to build directly a more complex 3D-object.
The other way is to create single surfaces to build a 3D-object.
Independent of the selected way to generate the CAD model, the result is called volume model, if all surfaces build a closed skin with a clearly defined inside and outside surface.