Whats the difference between xmpp.domain and xmpp.fqdn

Firstly I should mention im using Active Directory for authentication. I’m trying to figure out what if anything is the difference between the xmpp.domain and the xmpp.fqdn

The fqdn seems logical enough…I have it set to the fqdn of the jabber server: myjabber.mydomain.com

Should the domain be set the same as the fqdn or should it just be mydomain.com

If I’m using active directory how does the username get sent to the server? It seems like my username format has to be username@fqdn for me to authenticate?