I think we're discussing ID codes moreso than titles here. Many are based on production house and studio names, some might be more genre specific, others might just be simply available and unused elsewhere.
By way of the example, BBAN comes from Bibian as mentioned above - based on an odd transliteration of Vivian I assume (and probably should even be Vivienne!) Note that Bibian's compilations run separately under the BBSS code.
Other examples in the lesbian realm:
ARM, ARMD for Aroma Planning
CEMD, CESD, CETD for Celebrity Friends
NHDT, NHDTA, NHDTB for Natural High
Usually the big producers will run up to 1000 titles on a code and then move on to a replacement.
Sometimes production houses will run several simultaneously based on niche sub-genres, often using different studio labels. Mousouzoku is a good example, eg. EVIS on Ebisu-san (body part fetishism), SORA on Yama to Sora (dominance play) etc.