Classification systems and codes are key to understanding some census statistics. Many have changed over time and it is important to understand these changes when comparing statistics either from different censuses or surveys, or the same census or survey from different time periods.
Search for concordances or crosswalks to compare different classification systems or the same system over time. Not all systems will be comparable.
"The United States reports import and export statistics on a 10-digit classification system called the Harmonized System. The system is defined at the 2-, 4-, 6-, and 10-digit level for both imports and exports.
At a six-digit level, the Harmonized System codes are the same between imports and exports (and are the same as used by most industrialized countries of the world). At the 10-digit level, however, there are variations between imports and exports."
In the United States, the import classification system is called the "Harmonized Tariff Schedule of the United States" while the export system is called the "Schedule B".
Source: US Census Bureau/STAT-USA