And quoted from page 34/35 of the Iowa DOT truck guide
http://www.iowadot.gov/mvd/motorcarriers/truckguide.pdf :
"Registration weight is the tonnage declared on the license plate purchased for the vehicle and is purchased by the ton. Registration weight is sometimes referred to as license tonnage. The power unit registration must have a registration weight equal to or greater than the actual weight of the truck, trailer(s), and cargo; unless privileged to a registration weight exception. Registration weight exceptions are found in the registration weight tolerance and gross registration exceptions sections of this book.
For-hire carriers cannot claim registration weight exceptions and must always purchase registration weight equal to or greater than the gross weight of the truck, trailer(s) and cargo.
Examples: A truck-tractor/semitrailer combination with five axles and 51 feet of overall wheelbase is allowed a gross weight of 80,000 pounds, according to wheelbase Table 1 and Table 3. If this truck tractor is licensed for 40 tons (80,000 pounds), the combination may have a gross weight of 80,000 pounds. If it is licensed for 30 tons (60,000 pounds), it may only have a gross weight of 60,000 pounds, as it is limited by the license tonnage.
If a similar vehicle is licensed for 40 tons (80,000 pounds) but only has 48 feet of wheelbase and five axles, it will be allowed a maximum weight of 78,000 pounds, which is the weight limit established in the wheelbase tables."
License weight is a thing in Iowa, not sure how many other states. Better to check than be sorry.