Use the 'show_in_menu'
argument of register_post_type()
to specify the URL path of the parent menu item. For example, a value of 'edit.php'
will nest the CPT’s management pages in the Posts menu item, 'edit.php?post_type=page'
will nest them in the Pages menu item, and a value of 'users.php'
will nest them in the Users menu item.