The Explanatory Notes are the official commentary on the Harmonized System, published by the World Customs Organization. They provide detailed guidance on the scope, content, and structure of every heading and subheading at the international (6-digit) level.
In the United States, the Explanatory Notes are not legally binding but are widely recognized as persuasive authority. CBP, the Court of International Trade, and the Court of Appeals for the Federal Circuit routinely cite them when interpreting heading language. The U.S. Statement of Administrative Action accompanying the Uruguay Round Agreements Act describes them as "generally indicative of the proper interpretation" of the Harmonized System.
Each Explanatory Note describes what falls within a heading, what is excluded, and provides illustrative examples. They are published in multiple volumes and updated periodically by the WCO. The most recent edition is the 2022 Edition, reflecting HS 2022 changes.