In the United States, all imports are classified by a system of 10-digit product codes called the Harmonized Tariff Schedule. Exports are classified by a different system of 10-digit codes called Schedule B. Both systems are based on the international Harmonized System (HS), a 6-digit code in which each pair of digits represents a level of hierarchical classification. For example:
So for any given product, the first six digits of the Harmonized Schedule code will be the same as the first six digits of the Schedule B code. Only the last four digits, which break products down into even more detailed categories, will be different. Because of this, many databases that combine imports and exports will use only the first six digits. Others will convert one system to the other using concordance files.
Alternatively, some data products use Standard International Trade Classification (SITC) codes, an older 5-digit product-coding system created by the United Nations. (The SITC and the Harmonized System are inter-related, but the actual codes are quite different.)
Canada, Mexico, and the United States have also developed a new set of product codes called the North American Product Classification System (NAPCS) which concentrate specifically on "service products" not covered by the Harmonized System. A separate US system of Advanced Technology Product codes is updated annually.
Finally, because some people just can't get enough code systems, some foreign trade data sources categorize products by the industries which produce them, using SIC or NAICS codes.