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.
The class determines the cost of the shipping. The lower the class, the lower the cost.
If you don't include an NMFC code on your BOL, it is very likely that your carrier will charge you a reclassification fee and charge you the cost of shipping the freight they determine your shipment is.