Aids to Navigation vessels
The United States Coast Guard maintains several classes of of small Aids to Navigation vessels, in addition to its classes of larger buoy tenders.
It classes all its buoy tenders as cutters -- vessels large enough for the crew to live onboard for mission lasting several days or more. These vessels have a large deck, for carrying buoys, and a large crane, for deploying them, inspecting them, or removing them. Usually these buoys mark the boundary of the safe shipping lanes for larger vessels, like frieghters or cruise ships.
Because buoys, that mark the boundaries of shipping lanes for smaller craft, like pleasure craft or fishing vessels, operating out of smaller harbors, don't need to be as large as those needed for ocean-going ships, they can bed deployed, inspected and removed by smaller vessels. The smaller vessels deploying these buoys don't need to travel as far as the larger ocean-going buoy tenders, so the crew don't need accommodation to live on board.
The Coast Guard's aids to navigation vessels range in size from just 16 feet to 66 feet. Buoy tending cutters all have official names. Aids to navigation vessels, like the Coast Guards's motor lifeboats, and other vessels too small to accommodate their crew to live aboard, are known only by a five digit number. The first two digits in the number designate the vessels length.
