As stated on the International Trade Administration website, "The Harmonized System is a standardized numerical method of classifying traded products. It is used by customs authorities around the world to identify products when assessing duties and taxes and for gathering statistics." Often you'll want to know these codes if you're looking for import or export data such as that available via the UN Comtrade website. There are other classification systems too, however, such as SITC and BEC.
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