You're using a feature called "shared subscriptions", for which there doesn't seem to be much in the way of documentation.
It's a feature of SJSAS/Glassfish clusters, and is only available when using the jmsra resource adapter.
It allows each of the Glassfish instances to configure MDBs to receive messages from the *same* durable subscription. To achieve this they need to supply the same subscription name and clientID (if you deploy to the cluster this typically happens automatically).
The normal JMS restriction on duplicate clientID is not applied, so long as the "duplicate" clientIDs are in separate clustered instances.
Under the covers, the broker load balances messages to these multiple subscribers in much the same way as it does when delivering messages from a queue.
Nigel
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