The http://docs.jboss.org/jbossas/jboss4guide/r5/html/ch7.chapt.html says:

BlockingTimeoutMillis: This attribute indicates the maximum time to block while waiting for a connection before throwing an exception. Note that this blocks only while waiting for a permit for a connection, and will never throw an exception if creating a new connection takes an inordinately long time.

What does "a permit for a connection" mean? Is that something that comes from the database are something that comes from the container?