A Maritime Mobile Service Identity is a nine-digit number that uniquely identifies your boat, you the owner and your contact information. Each Digital Selective Calling radio needs to be registered and assigned an MMSI number. Your DSC radio can be used to send a distress alert to the Coast Guard and nearby vessels (with DSC radios in receiving range) or to selectively contact another vessel when you know its MMSI number.
If you have relatively new VHF radio in your boat, chances are it is equipped with Digital Select Calling or DSC. The most important feature of a VHF DSC radio is that you can send a Distress Alert which will tell the Coast Guard and other boaters in your area that you require immediate assistance.