@Override
        public String toString()
        {
            return "PubSubService.UniqueIdentifier{" +
                    "serviceId='" + getServiceId() + '\'' +
                    '}';
        }
A unique identifier for an item, in context of all nodes of all services in the system.

The property that uniquely identifies a service within the system is its serviceId.
xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/PubSubService.java