The National Motor Freight Traffic Association (NMFTA) created the National Motor Freight Classification (NMFC). It is used for all interstate, intrastate, and foreign commercial movement of LTL cargo. NMFC codes provide standardized freight classes to determine the transportability of the large variety of commodities that are shipped together in LTL shipments every year.
Freight class 50 is the least expensive because it refers to the freight that is easiest to transport for the carriers.
Density is the relationship between weight and size. A ping pong ball, which is very little material surrounding a good amount of air, is low density. Bricks, which are heavy for their size, are high density. In LTL shipping, it is commonly measured in pounds per cubic foot.