National Motor Freight Classification (NMFC) is the freight classification system that was created by the National Motor Freight Traffic Association (NMFTA) and is used for all interstate, intrastate, and foreign commerce movement of LTL cargo. NMFC codes provide standardized freight classes to determine the transportability of most of the countless different commodities that are shipped together in LTL shipments each year.
If the couch you are shipping has not been assembled yet and you are shipping various components that you can use freight class 250. If you are shipping a complete product then you should use freight class 175.
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.