This is the TCP port a WAM plugin uses to send authentication queries to its User Authority Gateway. An admin checks this against the firewall rules between the web server and the UAG, since a blocked or incorrect port here, even with the right hostname and IP, produces the same cannot authenticate symptom as the UAG being fully down. For example, port 18183, the default Check Point UserAuthority port, being blocked by an intermediate firewall rule change would explain a sudden authentication outage.