unique Only
Return a UniqueOnly delivery mode with a unique id for this fragment. In rare circumstances, if you make two identical subscriptions with the same (or all) properties in this fragment, provide a customId to avoid collisions.
Parameters
custom Id
An additional custom id to identify this subscription. Only necessary if there are two subscriptions in this fragment with exact same properties (i.e. two subscribes, or two selectSubscribes with the same properties).