It probably isn't very different (other that the "service pack" directory is predefined) except that it didn't work in my case. I wanted to fix a trivial bug in the webservices stack by replacing a single class in the webservices-rt.jar so I created my own jar with that only class and used the prefix to specified its path but, alas, that didn't do the trick. So, I ended up replacing that class in the original jar.
[Message sent by forum member 'vladchuk' (vladchuk)]
http://forums.java.net/jive/thread.jspa?messageID=242601