I would like to understand the difference and how aircraft manufactures designate the "sub" type of a paticular series. For example UA 747-422, LH 747-430 and AC 747-433 are all of 747-400 series, but what do the second and third digists (after - ) mean? I know they probably related to engine choices but how exactly are they assigned? sometimes a fourth letter such as F and M are added. I guess they mean freight and mixed. But are there any other sub categories? Is there such a database for quick look-up?
Many thanks in advance.