You make a list of all the locations you are willing to go to on day 1 or 2 of new hire training. You then get trained to answer phones in their call center. When a spot opens up at a location they offer it to each person (who requested it) in seniority order (which is by class start date, and within each class by age is your seniority). If you want it, you go. If you don't, they ask the next person.
A common question is "How long will I be answering phones in JAX until I get to my location", and the answer is nobody knows, and it changes all the time as people leave ATP. Could be a few days, could be 2 months if you will only go to 1 location and no one is leaving from there.