certificateServer is a DisplayString reporting the validity status of certificates used by the gateway, distinguishing an alert for all certificates expiring/expired, a warning for one or more invalid, and a disabled state when none are defined. An admin relying on certificate-based VPN authentication can use this to get advance warning before an expiring certificate breaks client connections, rather than discovering it only after logins start failing. For example, "Alert: Certificates Validity: All certificates are going to expire/expired;" gives the admin a clear trigger to renew certificates before the current ones lapse and lock out certificate-authenticated clients.