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.

Freight classification groups products based on similar density, stowability, handling and liability so carriers and shippers have an effective way of communicating the ease of transporting said freight.

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.