Where can I find the definition of a Schedule B number and a HTS number?

Schedule B is used to classify products for U.S. exports and is administered by the U.S. Census Bureau. You can search for schedule B numbers at https://www.census.gov/foreign-trade/schedules/b/index.html. For U.S. imports, the Harmonized Tariff System (HTS) is used, and it is administered by the U.S. International Trade Commission (USITC). You can search for HTS numbers at https://hts.usitc.gov/.