Two separate explanations that I found:
1 - "Under the U.S. Coast Guard Federal Boat Safety Act of 1971, boats less than 20 feet powered with an inboard, outboard, or stern drive engine manufactured after November 1, 1972, must display a capacity plate defining the safe load limits. This plate must be mounted where you can see it when preparing to get underway. Sailboats, canoes, kayaks and inflatable boats are exempt from this standard states do have statutes prohibiting the carriage of people and gear in excess of the stated capacity, or the installation of a motor that exceeds the recommended horse power limit. Also, you may void your insurance policy if you are found to have exceeded the limits stated on your capacity plate."
2 - "On August 1, 1973, the USCG effected a regulation, Title 33, Code of Federal Regulation, Section 183, requiring all monohull boats under 20ft. (except sailboats, canoes, kayaks, and inflatable boats) carry a plate posting the Maximum Weight Capacity, Maximum Persons Capacity and Maximum Horsepower Ratings. Boats constructed before this date may have these items posted but may not be in compliance with CFR 183.
These capacity plates can be found near the helm. These plates are yellow with a silver/gray background. Boats that are under 20 ft. have capacity plates labeled "U.S. Coast Guard Maximum Capacities". Boats up to 26ft carry just the "Maximum Capacities" wording.
Boaters are required by law, to obey these safe loading capacities posted on these plates. In addition to the USCG approved weight, load, and horsepower capacities, NMMA Certified boats also list other standards by which they are certified which may include navigation lights, flotation, maneuverability, compartment ventilation, steering, fuel, and electrical systems. If you see the NMMA logo and the words National Marine Manufacturers Association, you can be assured that the boat meets additional industry standards above and beyond the USCG regulations. "