Paper-making machinery (843920) covers equipment that forms the paper web from pulp — fourdrinier wires, press sections, and dryer sections. Finishing machinery (843930) covers downstream operations applied to the formed sheet, such as calendering, coating, and cutting to size. The functional stage in the production sequence determines the correct subheading.

Parts are split by the machine they belong to: parts for pulp-making machinery go under 843991, while parts for paper-making and finishing machinery go under 843999. If a part serves both functions and cannot be attributed to one machine type, classification follows the heading that corresponds to the machine's principal function, and a binding ruling may be warranted.