Background: Revit has a series of built-in Categories. Some of these categories can take directshapes, some can take loadable families and others can take system families. Many of the system family categories such as Walls, Floors, Roofs and Stairs tend to be very popular. By default, it seems that only in-place component families can be created in many of these system categories. That is not actually true.
By creating custom family templates, loadable families can be placed in these categories also. Here is a list of categories and what types of elements they take.
To find out how to open up these categories to new families, see the Revit: Types and Families guide