In the case of Boeing for instance, the first figure relates to the series. The remaining figures (or aplpha numerics) relate to the (original) operator ordering the aircraft. For example an American Airlines B757-223, the 2 denotes that itīs a 200 series, and the 23 identifies it as an American Airlines model (i.e. AAL specifications). Iīm not sure if itīs exactly the same with Airbus. I think that with them itīs more to do with a certain specification (engines/interior configuration etc) than any association with a specific operator.