Section, chapter, and subheading notes are part of the legal text of the HTSUS. Under GRI 1, classification is determined by the terms of the headings and any relative section or chapter notes, so a note can settle a classification before any later rule is reached. A classification that reads correctly against the heading text can still be wrong if a note in the section or chapter removes the goods.
Notes do different jobs, and reading one correctly starts with knowing which kind it is. Definitional notes fix the meaning of a term for a section or chapter. Inclusionary notes bring goods into a heading that its text alone might not cover, and exclusionary notes take goods out of a heading they would otherwise fall within. Cross-reference notes point to where excluded goods belong instead. Tariff-rate quota notes, usually Additional U.S. Notes, set the quantities and conditions that decide which rate applies. The terms here describe each type and how it operates.