The classification system that assigns every imported good a numeric code determining its duty treatment.
The Harmonized Tariff Schedule of the United States is the master classification system Customs uses to determine what duty applies to an imported good. Every product is assigned a 10-digit HTS code: the first six digits are the internationally harmonized "HS" code shared by most trading nations, the next two are set by the US International Trade Commission, and the final two are a US statistical suffix. The HTS code is the anchor everything else attaches to — the MFN base rate, USMCA eligibility, and any Section 301, Section 232, or AD/CVD coverage are all determined by which HTS heading a product falls under.
Maintained by the US International Trade Commission (USITC) under 19 U.S.C. 1202.
For a specific entry's rate, the free Tariff Calculator gives you the stacked, cited answer.