Rookie and Expert are the only divisions I have noticed.
In the "showdown rush" there are only two divisions (the most recent one was a "rush"), in the ordinary showdowns there are three. I think everyone is put automatically in Rookie but you can change it before you enter. There are various reasons to play in rookie, as I do myself.
@EchoUK: I've no intention of justifying myself to you so you may not feel like answering my questions, but; is it then ok for a Legend to play at rookie level? If you get promoted, will it be ok for a TL to play at rookie? Generally, should the rule be that only those at the same level as, or lower than, you can play at rookie.
You are the one breaking the rules. FYI, the person you named has played a lot more showdowns at veteran than he has at rookie.