When the text of an HTSUS heading or note is ambiguous, CBP and the courts use the same interpretive canons that apply across U.S. statutory law. This category covers four of them. Expressio unius supports an inference that listing some items excludes others; lex specialis prefers the specific provision over the general one; noscitur a sociis reads a term in light of the words around it; and mutatis mutandis, which appears in the text of GRI 6 itself, applies the rules at the subheading level with the necessary changes.
Canons resolve ambiguity; they do not replace the General Rules of Interpretation. Classification still starts under GRI 1 with the terms of the headings and any relative section or chapter notes, and a canon cannot override clear legal text. Canons matter most in close cases: protests, ruling requests, and litigation at the Court of International Trade, where how a single word or list is read can decide the heading. Each entry explains the canon and how it has been applied to tariff terms.