The National Motor Freight Traffic Association (NMFTA) publishes a list of freight class designations, codes, and subclasses for commonly shipped items (https://classit.nmfta.org/). A subscription is required to view this list. You can read more about freight classes 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.
Some items, such as car transmissions, have a specific class no matter how heavy or big they are. This is called a fixed class. Items can be assigned a fixed class if the value and transportability very rarely change.