An MMSI number lets you transmit your position and type of distress
When connected to a Global Positioning System (GPS) the radio will transmit the caller's position and type of distress and display it as an icon on the chart plotters of boats that receive the call. The system then determines the distance and heading to the distressed vessel, thus allowing nearby boaters to assist in cases where they are closer than the U.S. Coast Guard (please follow the instructions of the U.S. Coast Guard in all emergency situations.). Additionally, MMSI numbers allow boaters to connect directly to other boaters and marine assistance dispatchers with DSC-VHF radios, which will decrease the amount of traffic on commonly used radio channels.
When installed in your DSC-VHF radio, MMSI numbers can be used by the U.S. Coast Guard for search and rescue purposes within Rescue21 service areas.
If you plan to operate on International waters outside the United States, you must get your MMSI number through the FCC.