At Comair, the newhire class is broken into seniorty by their birthdate. Older = more senior. The class is given a list of available domiciles, and they start at the most senior person and work their way down. Some classes only get one domicile choice while others will have more options. After you get assigned a domicile as a new hire, you can bid to any domicile with openings when the vacancy bids come out. If you get awarded that domicile, it is a one-year lock in aircraft, base, and seat. If you get displaced into another domicile you can bid back to your current domicile as soon as there is an opening.