When you register interest in monitoring a designated sublist, you can specify 16 bytes of user data (called user notification controls) to be associated with the sublist. The use of the user notification controls (UNCs) depends on your application requirements. For example, the UNCs might contain information about the meaning of the sublist. If a sublist transition occurs (and an EMC is queued to your event queue), the EMC will contain the 16 bytes of user notification controls. The system returns this information to you when you read and dequeue the EMCs by issuing the IXLLIST REQUEST=DEQ_EVENTQ macro.