The Main Menu is a component which can have sub panels. It is referenced by the Display Panel Manager such that you may easily open and close it.
Learn more about the Main Menu component here. The Edit sections allows you to find the main menus sub panels and the buttons that opens them. To Add panels to the Main Menu simply select the “Main Menu” option in the Panel Options when creating one of the available panels (Inventory Grid, Save Menu, Crafting Menu, etc.)