Almost all U.S. LTL carriers use NMFC freight classes, although some are attempting to move to a dimensionally-based system.
Your freight cost is determined by a number of variables, such as how far your shipment needs to go, the freight class number, whether or not accessorials are needed, as well as fluctuating fuel costs and truck capacity.
NMFC freight subclasses are used to differentiate among commodities of the same type. For example, one NMFC code for chairs/stools is 82790. Subclasses 82790-1 and 82790-11, despite have the same base NMFC code, fall into two different freight classes.