This is a notification-carried value giving the username the system was trying to authenticate when the TACACS+ server transitioned to down. Admins use it, in traps, to see which specific authentication attempt was in progress during a server outage, though it's only populated when the transition was due to a packet-level timeout. A trap carrying rbnTacacsUserName of "jdoe" during a server-down event tells the admin that user's login attempt was in flight when the TACACS+ server stopped responding.