Freighter identification code
From Halopedia, the Halo wiki
Freighter identification codes usually compose of a three-letter code followed by an eight- or nine-digit number. The letters correspond to the first letters of the freighter's name. If the freighter's name is only two words, the letter "X" is used as a substitute for the third letter (e.g. Contents Under Pressure would be "CUP", while Bulk Discount would be "BDX"). A notable exception to this is the Wholesale Price, though this may be an oversight.
List of known freighter identification codes
List of appearances