Every month the company publishes a bunch of different schedules to choose from. Some schedules are good for people who commute, and other schedules are good for people who live at their domicile. Most schedules aren't good for anybody

. Every month you bid on all of these different schedules by telling the company which one you want the most, etc. The more seniority you have at the company (the longer you've worked for them), the better your chances of getting the schedule you want.
Bidding also applies to which domicile you are assigned and in most cases which airplane you get to fly.