This is from the NCAA Tournament Handbook (I posted a link somewhere before).
All suggested officials are eligible for First/Second round games.
Anyone who works a First/Second round game is eligible for a Regional Semifinal/Final.
Anyone working a Regional is eligible for the Final Four. Ten officials are selected - three for each game, one alternate to serve at all three games. (In the earlier rounds, the alternate official of each game is the R from a different game).