The Message Precedence Section:

Message Precedence is used to assign priority to messages entering the queue. By default, mail is considered "first-class mail," and is given a precedence of 0. The higher the precedence number, the higher the priority of the message.

But don't get excited. Increasing priority is essentially meaningless. About the only useful thing you can do is to select a negative precedence number, which indicates low-priority mail. Because error messages are not generated for mail with a negative precedence number, low priorities are useful for mass mailings. The precedence values from the Red Hat sendmail.cf are

Pfirst-class=0
Pspecial-delivery=100
Plist=-30
Pbulk=-60
Pjunk=-100

Precedence values have very little importance. To request a precedence, mail must include a Precedence header, which it very rarely does. The five precedence values included in the sendmail.cf file that comes with your Linux system are more than you'll ever need.