I've been assuming it's their creche designation, where the stolen babies were raised together and trained, and there's a whole lot of age-mates in each at the start but as the kids grow and are either not useful to the First Order and are 'retired' from the program or killed in training/action, the cadre shrinks. Finn's was down to 4 in Before the Awakening, so if the FN group started out at any sizeable number, there was a high failure rate.
Re: Stormtrooper designations specialties?
(Anonymous) 2016-03-14 04:01 am (UTC)(link)