HSN stands for Harmonized System of Nomenclature. It is a system for classifying commodities created by the World Customs organization. U.S. LTL shipping, however, uses NMFC classifications instead.
No. If you claim that your freight is a lower class than it actually is, your carrier will likely find out. They will charge you a fee for reclassifying the freight and charge you the appropriate amount for the actual class of your freight after it has been shipped.
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.