A Revit family type built into the project itself - walls, floors, roofs, ceilings - that can't be loaded, saved, or shared as a separate file the way a loadable family can. It's the fundamental split every new Revit user has to learn: loadable families (like a door or a piece of furniture) live in RFA files and get loaded into a project, while system families are always project-hosted, copied between projects via transfer project standards instead of a family load.