626X is a Limited Duty Officer (LDO) in the US Navy. LDOs are experts and leaders in their Navy enlisted rating or Marine Corps enlisted MOS. They are considered more of an officer and less of a technician than a chief warrant officer (CWO).
The 626X designator identifies a Limited Duty Officer (Ordnance - Submarine). Officer designator codes are four-digit numbers used to group officers by categories for personnel accounting and administrative purposes.