All of the imports and export codes used by the United States are based on the international Harmonized System (HS). The HS assigns 6-digit codes for general categories. Countries which use the HTS are allowed to define commodities at a more detailed level than 6-digits, but all definitions must be within that 6-digit framework. The U.S. defines products using 10-digit HTS codes. Exports codes (which the U.S. calls Schedule B) are administered by the U.S. Census Bureau. Import codes (called HTSA) are administered by the U.S. International Trade Commission (USITC).
Click here for translating a HS code into its corresponding SITC & NAICS codes or NAICS & SITC codes into HS codes.
Click here for correspondence between NAICS and SIC codes
For more international economic and social classification systems please click here.