I would like to know what $_registered_pages does and where am I supposed to use it.
p.s was unable to find any documentation on it online.
$_registered_pages is a temporary container for all registered admin menu pages. It is filled in
add_submenu_page() and used used later in
The leading underscore indicates you should not use it for anything. It is private and might change its name any time without warning. So do not use it in your own code, that will break one day.