http://www.ietf.org/rfc/rfc3348.txt
Many IMAP4 [RFC-2060] clients present to the user a hierarchical view of the mailboxes that a user has access to. Rather than initially presenting to the user the entire mailbox hierarchy, it is often preferable to show to the user a collapsed outline list of the mailbox hierarchy (particularly if there is a large number of mailboxes). The user can then expand the collapsed outline hierarchy as needed. It is common to include within the collapsed hierarchy a

visual clue (such as a "+") to indicate that there are child mailboxes under a particular mailbox. When the visual clue is clicked the hierarchy list is expanded to show the child mailboxes.

Several IMAP vendors implemented this proposal, and it is proposed to document this behavior and functionality as an Informational RFC.

There is interest in addressing the general extensibility of the IMAP LIST command through an IMAP LIST Extension draft. Similar functionality to the \HasChildren and \HasNoChildren flags could be incorporated into this new LIST Extension. It is proposed that the more general LIST Extension draft proceed on the standards track with this proposal being relegated to informational status only.

If the functionality of the \HasChildren and \HasNoChildren flags were incorporated into a more general LIST extension, this would have the advantage that a client could then have the opportunity to request whether or not the server should return this information. This would be an advantage over the current draft for servers where this information is expensive to compute, since the server would only need to compute the information when it knew that the client requesting the information was able to consume it.