The maximum sessions property determines how many messages an MDB can process simultaneously. The default value for this property is 1, which means that an MDB must finish processing the first message on the queue before it can process the second, and so on (for you programmers, this essentially means that an MDB is single-threaded).
If the application server is not under a heavy load, you might want to increase the throughput of messages. In this situation, you can increase the value of the maximum sessions property to enable an MDB to process more than one message at once (this makes the MDB multi-threaded). It is worth noting, though, that increasing the value of maximum sessions will increase the load on your system, since you will now have multiple instances of the MDB running at the same time!
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