Per XEP-0368, "Provides users a virtually zero overhead way to bypass restrictive firewalls that only allow HTTP over port 80 and HTTPS over port 443. For servers, it allows multiple services to run on the same port."
https://xmpp.nasqueron.org/dns-check.jsp should help to configure this according https://github.com/igniterealtime/Openfire/pull/1450 but it's not clear what's the actual effect