This is the National Form of Registration for your boat. It's the "official number" awarded to each documented boat that stays with the boat forever. It does not change as the boat is resold over the years from one owner to the net. Vessels that measure at least five net tons are required to have a Documentation Number. One thing to understand is that "five net tons" does not refer to the weight of the vessel, it is associated with the vessel's volume. In simple terms, vessels that are more than 25 feet in length will measure five net tons or more.
Any vessel at least 5 net tons (roughly 25'+) wholly owned by a US Citizen used in fishing activities on navigable waters of the US or in the Exclusive Economic Zone (EEZ)