You will be given a so-called "default" RAP beginning at 0400 on your first day of reserve. This is not spelled out in the contract, is pretty obviously not contractual, but the company convinced a friendly arbitrator to sign off on it so that's what we all get.

You can, however, proffer/preference out of 0400 RAP. The day before a reserve day, you look at the open trips and the reserve list and between 1000-1400 CT submit your proffer/preference for the next day. If your seniority can hold what you asked for you get it, and can confirm it after 1700. If you either don't get what you asked for, or fail to (or decide not to) confirm it, you default back to 0400 RAP.

In reality you will get a mix of flying, airport standby, and RAPs, different every day. As you get some people under you, you'll get what you proffer for more often. If you can't get something with a late sign in, you'll have to commute in the night before.