System families contain family types that you use to create basic building elements such as walls, floors, ceilings, and stairs in your building models.
System families also include project and system settings, which affect the project environment and include types for elements such as levels, grids, sheets, and viewports.
System families are predefined in Revit LT and saved in templates and projects, not loaded into templates and projects from external files. You cannot create, copy, modify, or delete system families, but you can duplicate (copy) and modify the types within system families to create your own custom system family types. You can delete all but one system family type in a system family, because you need at least one type per family to create new system family types.
Although you cannot load system families into templates and projects, you can copy and paste or transfer system family types between projects and templates. You can copy and paste any number of individual types, or use a tool to transfer all types in the system families that you specify.
System families can also host other kinds of families, usually loadable families. For example, a wall system family may host a standard door/window assembly.