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.
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.
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.