The mail address contains the name of the recipient and the system where the mail message is delivered. When you are administering a small mail system that does not use a nameservice, addressing mail is easy: Login names uniquely identify users.
Mail addressing for more complex sites is also easy when you use DNS Mail Exchange (MX) records.
UUCP route-based addressing requires the sender of an e-mail message to specify not only the local address (typically a user name) and its final destination but also the route that the message must take to reach its final destination.
UUCP route-based addressing is shown here for historical reasons. These days, almost no one uses this form of addressing.