A Heading is the four-digit classification level within a Chapter of the Harmonized Tariff Schedule. It is the most important level for classification because under GRI 1, classification is "determined by the terms of the headings."
For example, heading 8471 covers "Automatic data processing machines and units thereof" — that is, computers and computer parts. Below the heading sit one or more subheadings that further specify the product.
Headings are the foundation of every classification decision. Customs rulings, court cases, and Explanatory Notes primarily interpret heading-level language. When competing headings could apply, GRI 3 provides the tiebreaker rules.