This counts RADIUS responses from this server whose authenticator or message-signature attribute failed validation, the specific symptom of a shared-secret mismatch between the switch and the server. Because this is checked against the shared secret configured in the AuthKey or AuthSecret objects, this counter climbing on just one server entry while others stay at zero is close to conclusive evidence that this particular server's secret was rotated on one side but not the other. For example, if a security team rotates the RADIUS secret on the AAA server for compliance but the switch team doesn't push the matching update, every 802.1X login against this server will increment this counter even though the server is reachable and responding.