As far as I know, core-threads is the # of threads allocated at startup.
For more info regarding these properties, you can read the following:
That's what we expected, but discovered that the threads NEVER grew to the Max. They remained set at core-threads; which was surprising. I.E. Our max is 300 and we set start to 36 (which aligned to the number of queues we have in the Queue Managers, as we'll pretty much have at least one MDB firing regularly). We expected the threads to grow as queues built up and MDB's fired. However, looking in the JBoss console, we only ever see 36 and a backlog begins.