Almost all U.S. LTL carriers use NMFC freight classes, although some are attempting to move to a dimensionally-based system.
Every commodity has both a freight class and a NMFC code. Freight class represents a category of items while NMFC codes relate to specific commodities within each of the 18 freight classes.
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.