Each item is a menu is in effect a sub-menu (just all the rest of the standard menus). The system will however use the first one (whatever is the lowest ID number) as the main menu label and link, again, just the same as the standard menus.
Every menu must have a title and a link. It really is no more complex than that. Add a menu – give it a title, give it a link address and save. Perhaps you are trying to over-think it. Don’t forget, I did always call the add-ons “simpl_”.