@jcbrand in https://social.broers.me/display/24324cc46cafbb0f42c1cc431d89cf0df62ee5bd the meaning of "XMPP URI of Associated Publish-Subscribe Node" is being discussed. You are asking for the XEP, which this comes from. It's XEP-0045, apparently and the text is exactly the same in there, but I still have no clue about it if or how to make use of that setting. By the nature of pubsub I would assume that clients may subscribe to that and get notified of new messages when they happen to be not in that room...