Maritime Mobile Service Identity number
A series of nine digits that uniquely identifies a vessel or shore station for all communications with maritime purposes. MMSI numbers are sent in digital form over a radio frequency to make a call to a specific vessel, or as a group call to a group of vessels. The numbers are issued by authorities in the country of ship registration and are required for all vessels operating at high seas in keeping with the Global Maritime Distress and Safety System recommendations (GMDSS).