The many definitions or classes of freight are defined in the National Motor Freight Classification tariff, also known as the NMFC.
The NMFC is a publication for motor carriers containing rules, descriptions, and ratings of all commodities moving in commerce. The publication is used to classify freight for freight bill rating purposes.
Besides defining the classes of shipping commodities, the NMFC also assigns item numbers to each type of commodity. The item number is related not only to the commodity itself, but to its packaging, the material from which the commodity is made, and other considerations. Item numbers are associated with rates as well as commodity classifications.