To my knowledge it's different in each weight in each class- there's no set way to determine it. Also, if memory serves me correctly (ahh, Chairman Kaga), they don't announce that until after regionals.
it's all drawn. each weight class is different. They don't come out with the matchups until after regionals are over so people can't see where they would fit in best and try to throw a match to get second, if they thought it would be a better side of the bracket. The system could match up #1 and #2 ranked kids in the semi's. That is why some state championships are won in the semifinals. examples would be regional A winner wrestles Regional B #4. The regional names come out after midnight Sunday after regional Saturday.