Each Aircraft Unit Load Device (ULD) is identified by its ULD code. This code is a unique combination of letters and numbers. The codes such as LD2, LD3, LD9, M1 etc. were developed under ATA many years back, while the codes such as AKE, AAP, AMA, RKN, VGA are part of the IATA ULD identification system. In general the use of IATA Identification system codes is preferable.
ATA: Air Transport Association of America
IATA: International Air Travel Association