Looking through flightradar24 and I've noticed that there's loads of different 747-400 variations:
What do the variations mean?
As others have said they are customer codes that stick with the aircraft for it's entire lifetime. They show who originally ordered the aircraft. Not OPERATED but ORDERED. i.e. an aircraft ordered by Malaysian but never taken up by them and sold right away to Lufthansa would not have been delivered with a customer code of 30 but with a customer code of H6.
"In 2016, Boeing announced that they would no longer apply customer codes to any aircraft produced after a certain point, which would lead to their designators being the "generic" type for the model. The codes were removed from the type certificates for each model with effect from the production line number shown below:
Boeing 737 Next Generation: line number 6082
Boeing P-8 Poseidon: line number 6020
Boeing 747-8: line number 1534
Boeing 767: line number 1102
Boeing 777: line number 1422
Further, customer codes have never been used for the 787 and 737 MAX, and will not be applied to the 777X either"