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 class 50 is the least expensive because it refers to the freight that is easiest to transport for the carriers.
Yes. If your shipment can be stacked, it allows the carrier to fit more freight into their truck.