Many people come up with names for different things that sometimes confuse us in many ways.
Yours for example:
pre-fab: a file that can be loaded over and over again into many projects without causing a complete replacement of a scene but simply a merge into the scene.
Files in 3d studio max that are filtered as .3ds are what your calling pre-fab (slang). Like Jay Lundy said above me, a chair or a table or anything close to that nature. Can be almost any kind of geometry or shape or an edited shape/geometry into a full mesh.