FogBugz uses a service running on the server at all times to check email, perform background full-text indexing, and many other periodic maintenance tasks. This is the URL that service uses to communicate with FogBugz. In most cases, the default is OK: it's the same as the URL where you access FogBugz. You only need to change this in rare cases: if you have an unusual DNS configuration, like a load balancer, or if the FogBugz server itself doesn't have access to FogBugz through the normal URL.