An organization called the National Motor Freight Traffic Association (NMFTA) publishes a list of freight class designations, codes, and subclasses for many frequently shipped items (https://classit.nmfta.org/). To view this list, you must pay a subscription fee. Learn more about freight classes from the experts at Koho for free on our freight classes pages.
Adding NMFC code to a shipment is technically optional, but the NMFC codes can help avoid reclassification and ensure you're using the correct freight class number.
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.