Just to clear up further confusion:
- The first match you play will be the current round's starting tier. For this round it is ADV OU.
- The loser of the first match can then pick one of the remaining two generations to play the next match in. For this round, they can choose from RBY OU and GSC OU.
- If the set goes to a third game, the last match is played in whatever generation has not been chosen.