The purpose of a Harmonization Code (also referred to as Tariff Code, Customs Tariffs, Harmonized Tariff Codes, and Harmonized System Code) is to expedite the shipping process for international orders, as they are used for tax assessment purposes. This is a system used to trade most of the merchandise that is sent internationally and is a universal economic language and code for your goods.
Once an order reaches an international border, these codes will help the customs office to identify what it is that you are shipping. So it is recommended to have your harmonization codes listed and properly-identified at time of label creation.
For a full list of Harmonization Codes, head here.