This text describes how network names are used in XMPP. The intended audience is people looking to set up an XMPP-based service.
An XMPP domain consists of 1 or more XMPP servers. If there are more servers in the same domain, then that's referred to as "clustering".
When multiple XMPP domains communicate with each-other, at least one server of each domain connect to each-other. This is typically referred to as "federation" or "server-to-server comunication."