Unlike system families, loadable families are created in external RFA files and imported (loaded) in your projects.
Loadable families are families used to create the following:
Because of their highly customizable nature, loadable families are the families that you most commonly create and modify in Revit. For families that contain many types, you can create and use type catalogs, which allow you load only the types that you need for a project.
When you create a loadable family, you begin with a template that is supplied in the software and contains information about the family that you are creating. You sketch the geometry of the family, use parameters to establish relationships between family components, create the variations or family types that it includes, and determine its visibility and detail level in different views. When you finish the family, you test it in a sample project before using it to create elements in your projects.
Revit includes a library of content in which you can access loadable families that are supplied by the software and save the families that you create. You can also access loadable families from various sources on the Web.
You can load instances of families in other families to create new families. By nesting existing families inside other families, you can save modeling time.
Depending on how you want instances of these families to act when you add them to your projects (as a single element or as individual elements), you can specify whether the nested families are shared or not shared.