It's lottery with rankings. You pick your sites first, then your preceptors after sites are picked. Historically, 100% of 3rd years get one of their top 4 tracks (12 tracks total with half being very similar to another track just with some rotations flip/flopped).. Meaning 2 tracks are close to identical but like flipping peds/ob in blocks 5/6, for example. So if you rank your top tracks together you've got a very good shot at getting what you want.
4th years have gotten top 3 tracks (100%) for the past 7 years. 4th years have priority and schedule before 3rd years so that they can setup auditions/sub-i's.