As you already know, there are three kinds of families in Revit®: system families, model-in-place families and external (loadable) families (also know as .RFA files).
Some Revit® categories are exclusively system categories (families of those categories can be created only in project environment), for example: Walls, Stairs, Floors and similar categories. There are, however, hybrids (can both have system and external families in the same category), for example Structural Foundation categories but they are topic for some other time. Read More →