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.