Menu items have the same fields and structure in db as any post type. So, data for menu items is stored with the same logic as it is done for post types. You don’t need add any special words near ID in second param of get_field / the_field functions. You can get values anywhere, just need to know menu item ID ( not “object_ID” ). But, mainly, I believe, it will be used in a Walker, for example for creating some Mega Menus.