Many imports are components rather than finished articles, and the HTSUS handles them through a layered set of rules. A part is generally an integral, constituent component that the article it belongs to needs in order to function as designed; an accessory adds to or improves that article but is not essential to its function. The distinction matters because some headings provide for parts but not accessories, while others cover both.
Before either test applies, a component that is specifically described in a heading of its own, such as an electric motor, a bearing, or a screw, is generally classified there rather than as a part of the machine it goes into. Section and chapter notes set the order of priority (Note 2 to Section XVI is the best-known example), and parts of general use as defined in Section XV are excluded from most parts provisions in Sections XVI and XVII. Parts of parts raise a further question: whether a provision for parts of an article also reaches components of those parts. Because parts provisions often carry different duty rates and additional duties than the heading for the finished article, read these terms in the order they are applied.