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.
If you don't include an NMFC code on your BOL, it is very likely that your carrier will charge you a reclassification fee and charge you the cost of shipping the freight they they determine your shipment is.