A complex system used by the Ground Tranportation Inudstry to calculate charges for a shipment. There are divisions of what are known as Classes for various Articles of Freight. The lower the Class generally indicates a greater density. The lowest Class is Class 50, so igots of lead would travel the US Highway system as Class 50.
Now you're asking 'so what?' Class 50 prescribes a table of rates based upon the weight of the shipment. Before computers Desk Top Storage Space was limited and the rates couldn't be divided by the single whole pound or 10 pounds units. Rates are divided based upon per hundred pounds per article shipped. This system tended to keep rates schedules small (confined to 1500 page books) which in turn kept them handy for the clerks 'rating' the bill of lading.
Last thing considered in applying charges to a shipment using this system of Class Rates is the Diastance travelled. The farther it goes the more diesel is used, the more money the driver wants, and Fuel Tax is paid. So, it should cost more to ship something from Los Angeles to New York City than to ship From Los Angeles to San Francisco.