Solid - These models define the volume of the object they represent (as a stone). These are more realistic, but more difficult to build. Fixed models are mostly used for simulations in medicine or engineering, for CAD and specialized visual applications, such as ray tracing and constructive body geometry, as they have a substantial mass or material they represent (stone, metal object, etc.).

Shell / boundary - these models represent a surface, e.g. the boundary of the object, not its volume (like an infinitely thin eggshell). These are much easier to work with than fixed models. Almost all visual models used in games and movies are surface models.