Ok as the former CCC of NPTU Ballston Spa (and former detailers assistant), here's how it works:
At NY, the students would fill out dream sheets their 1 or 7th week of training (depends on when you got there, we changed the times up. ELTs were a little different). I would take these things and condense the sob stories on why students needed to go to San Diego (joking, I actually added a few words when needed to help get you what you wanted). Around the 18th week, NR would send out quotas detailing percentages per rate for subs and surface. We called these the "splits".
We would then take the dream sheets and count out what we already had as volunteers and what was needed. If we had excess subs, then we "force converted" you to surface. We would first look for "volunteeers". These were namely the folks that said "No longer desire subs". Next we took those that wanted homeport more than sub. After that we kinda rolled the dice. We took some upper half kids and some lower half kids and mde them surface (this was determined by your merit code). The higher the better would help us and the detailers get you what you wanted.
Note Merit code: 4 points for NFAS, 4pts for NPS and 2 points for NNPTU.
Flip side: Short on subs, we would pull service records for the class and look for everyone that had vol'd and put together a list. This list would be sent over to Medical for screening. Anyone that passed and did not say they wanted subs on the dream sheets, suddenly found themselves re-vol'd.
BTW, these splits are why orders were typically late from the detailers. Sometimes NR would wait until week 22 to send them to us.
Cheers and hope this helps.
Vinnie B