This read-only DisplayString reports the rejected user's address, containing a MAC address string for 802.1X users or an Inet address for login-type users, per its own description. It matters because it gives an admin the actual client identifier tied to the rejection, in whichever form is appropriate to the access method involved. For example, an admin tracking down a rejected 802.1X device would read this field as a MAC address and match it against the organization's asset inventory.