This is the shared secret specific to the dedicated user-login authentication servers, independent of whatever key is configured on the generic RADIUS client table. A secret rotation on the general-purpose RADIUS setup does not automatically apply here, so forgetting to update this one specifically would break only CLI or administrative logins while leaving other RADIUS-dependent features unaffected. For example, rotating the shared secret only on the general client table by mistake, while forgetting this user-login-specific key, would leave administrator CLI logins failing even though other RADIUS functions on the box work fine.