Yes, you bid for AM/PM/LCR and days off. It's hard lines.

Your days off are set, you wont get put on a three day on the last day of reserve. They use a "bucket" system. 1 day, 2 day, 3 day, exc. You can only get called out for the bucket you are in or less. Sometimes they will ask if you can cover a trip over a day off for 1.5x or in exchange for a different day later in the month, you don't have to do it. Sometimes this is a decent way to alter your schedule.