The 2 kinds of families in Revit LT are system families and loadable families.
Most elements that you create in your projects are system families or loadable families. Loadable families can be combined to create nested and shared families.
System families create basic elements that you would assemble on a construction site, such as walls, roofs, and floors.
System settings, which affect the project environment and include types for levels, grids, drawing sheets, and viewports, are also system families.
System families are predefined in Revit LT. You do not load them into your projects from external files, nor do you save them in locations external to the project.
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 LT. Unlike system families, loadable families are created in external RFA files and imported, or loaded, in your projects. For loadable 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.