Form factor describes the size of the projector:

Portableprojectors are small and light in weight, making them easy to transport.

Desktop projectors are larger than portables and made to be placed on a stable flat surface. They are not easily transported, but can be moved with some effort.

Stationary projectors are the largest and can be placed on either a vertical surface or attached to the ceiling using special mountings. These are not intended to be moved or transported once in place.