This counter records outright authentication denials received from any TACACS+ server used for login, meaning the server reached the user but explicitly refused the credentials rather than the request simply failing to arrive. A sudden spike here, as opposed to a spike in timeouts, tells an admin the TACACS+ server is up and answering, but users are supplying wrong passwords or the server-side account is locked, which redirects troubleshooting toward the identity store rather than the network. For example, a spike coinciding with a recent Active Directory password-expiration rollout would show up here first.