Menuitem class describes a single menu item. You can use all of the gadtools menus features expect Image menus here.
Since Menuitem class is a subclass of Family class, you can add other menu items as children of a menu item to indicate sub menus. MUI does not limit the level of sub menus, but the operating system currently allows a maximum nesting level of one. Because of this, children of menu items should not contain other menu items for now, the results are unpredictable.