623X is a four-digit number used by the U.S. Navy to identify a Limited Duty Officer (LDO) with an Engineering/Repair specialty. The Navy uses these officer billet designator codes to identify the primary naval specialty.
The Navy also uses naval ratings and NEC designators, which are similar to the Military Occupational Specialty (MOS) designators used in the U.S. Army and U.S. Marine Corps.