This is the number of times an IS-IS control PDU with the correct auth type has failed to pass authentication validation. It is the per-circuit breakdown of ciiSysStatAuthFails, meaning the authentication type matched but the actual key check failed, so a climbing value isolated to one circuit points at a stale or mistyped shared secret with that specific neighbor. For example, rotating the MD5 key on this router but not yet on one particular neighbor would show this counter climbing only on the circuit connecting to that specific neighbor while other circuits with already-synchronized keys stay flat.