From UFStarfleet Wiki
The Chief of the Boat is the Senior Enlisted Crewman on a starship or station of United Federation Starfleet. He/she assists and advises the commanding officer and executive officer on matters regarding the good order and discipline of the crew. There is only one COB on a vessel and he is generally responsible for the day-to-day operations of the boat's non-commissioned personnel and for their morale and training. While it is usually true that the COB is the most senior enlisted man, it is not strictly required. The CO is neither required to select the highest ranking man or the most senior in time in grade aboard, nor to replace his COB with a more senior man who reports aboard after the COB's selection.
When a new enlisted crewmember joins a ship's crew, the COB is usually one of the first people he will meet. While the COB is outside the direct chain of command for crewmembers junior to him, he has a tremendous amount of authority and carries a lot of weight, generally being thought of as the top of a virtual enlisted chain of command.