Almost all U.S. LTL carriers use NMFC freight classes, although some are attempting to move to a dimensionally-based system.
It might be tempting to declare that your shipment is a lower freight class than it actually is in order to secure a lower price, however, carriers will re-classify your freight for accuracy and charge you a fee for having to do so.
NMFC freight classes are determined using four factors: density, handling, stowability, and liability. All of these factors affect the cost to the carrier of shipping goods.