Since each commodity receives a unique National Motor Freight Classification, or NMFC, code from the National Motor Freight Traffic Association, no single code is more common than another. Each number correlates to a freight class, then used by shippers and carriers in calculating shipping charges.
Weight, dimensions, density, value, ease of handling, and liability determine the classification of freight. The NMFTA groups products into 18 freight classes, ranging from 50, the least expensive, all things constant, to 500, the most expensive. Commodities in Class 50 weigh more than 50 pounds per cubic foot, while Class 400 items typically weigh less than 1 pound per cubic foot.