When editing a Mastery Profile, you can add one or more Modules. Modules hold Topics. 

Mastery Profile context panel, Modules tab. The Mastery Profile is called World Geography. Three Modules are listed: Geography of Africa, Geography of Asia, and Geography of North America. On the right side of each Module name is a down arrow (to view more details) and an X (to remove the Module). Below the list is a +Module button.