The list of mailbox attributes in this document is very misleading. In particular the explanation of noinferiors is just wrong. It does not mean that the mailbox currently has no children, it means that it *cannot* have children ever. Also, it is untrue that marked and unmarked are only used by UW-IMAP. They are in the official IMAP specification and are used by at least Courier-imap as well.
One thing to watch out for, however, is broken IMAP servers which do send \Noinferiors when they mean that there are currently no children.
From the IMAP4rev1 specs (RFC 2060):
\Noinferiors
It is not possible for any child levels of hierarchy to exist
under this name; no child levels exist now and none can be
created in the future.
\Noselect
It is not possible to use this name as a selectable mailbox.
\Marked
The mailbox has been marked "interesting" by the server; the
mailbox probably contains messages that have been added since
the last time the mailbox was selected.
\Unmarked
The mailbox does not contain any additional messages since the
last time the mailbox was selected.
