Domain rewriting rules are used to convert addresses into true domain addresses and to determine their corresponding channels. These rules are used to rewrite addresses appearing in both the transport layer and the message header. The transport layer is the message's "envelope", which contains routing information and is invisible to the user.
The rewrite rules and the table of channels cooperate to determine the disposition of each address. The result of the rewrite process is a rewritten address and a "routing system"; i.e., the system to which the message is to be sent. Depending upon the topology of the network, the routing system may only be the first step along the path the message takes to reach its destination or it may be the final destination system itself.
After the rewrite process has finished a search is made for the routing system among the channel portion of the configuration file. Each channel will have one or more host names associated with it. The routing system name is compared against each of these names to determine to which channel to enqueue the message.
Note that PMDF provides other means of manipulating addresses for the purposes of changing them. See for instance Chapter 3.