This reports whether this specific call used the closed user group facility, an X.25 feature that restricts which parties are allowed to call each other regardless of the network's general call-routing rules. An admin checks this to confirm a call that was supposed to be restricted to a private group of users actually invoked that restriction. For example, seeing requested here on a call between two members of a corporate closed user group confirms the isolation feature was actually active for that specific connection, not just configured but unused.