It looks like if you are a 5A you go first. Then the 5B's, which are done by mileage class and then fare class.

I was a 5B on Friday being a 1k and they called about 20 people for stand-by. All of these were 5A's. So being a 1k I got to spend an enjoyable 3 hours on the floor of MCI, since my actual flight was delayed and I couldn't get on either of the two flight that left before hand to DEN.