Hi! This was well answered by the other person but I thought I'd add a few notes (current student). Every student will still apply via ERAS regardless of whether you take the spot that NYULISOM accepted you for so technically everyone applies the traditional match route. The school will actually encourage you to apply out too, to get a feel for the interview process/other residencies, and if you want to match to NYULI then you just rank them high on your list. There is no "locking in" to a field as you mentioned, the leadership emphasizes that whenever a student finds a specialty they like to contact them ASAP so that they can arrange for a specialty mentor in the respective field.
There are some current M3s that are applying to specialties outside of the 4 (gen surg, ob/gyn, peds, IM), I believe some are applying urology and IR. We won't know how successful they are yet till later this year, unfortunately. Good news is, research and other leadership roles are very bountiful here; there are spreadsheets of research opportunities and hardly and competition (perks of a small school). Residents and attendings are very open to having medical students join their team, all you need to do is shoot an email. Hope this helped!