An organization named the National Motor Freight Traffic Association (NMFTA) publishes a list of freight class designations, codes, and subclasses for frequently shipped commodities (https://classit.nmfta.org/). You must pay a subscription fee to view it. Find more information about freight classes from the experts at Koho for free on our freight classes pages.
Freight class 50 is the least expensive because it refers to the freight that is easiest to transport for the carriers.
No, not all commodities have freight subclasses, although many do.