NJE provides a store and forward file relaying service. Hosts are registered in a flat name space --- all users have names of eight characters or less and all hosts (systems) have simple names of eight characters or less. No provisions are provided for accessing hosts outside the NJE name space. All routing is implicit; each system knows how to reach (via a routing table) all the other systems on the network.
Mail messages usually correspond to class M NJE files. Files in this class are automatically processed as mail messages and not as regular files. PMDF is not normally concerned with other classes of NJE files; the selection of what classes of files PMDF processes is made by the NJE service and not by PMDF itself.