When a part is designed for use with multiple types of parent machines covered by different headings, classification generally follows the principal use or the machine for which the part is primarily marketed and sold. Importers should document the primary application and, where possible, reference the parent machine's model or specification. A licensed customs broker can help determine whether the part's design characteristics point to a single parent-machine heading.

Pressure cylinders specifically designed and presented as replacement parts for fire extinguishers are generally classified here as parts of fire extinguishers. However, if a cylinder is a general-purpose pressure vessel not dedicated to extinguisher use, it may be classified under pressure-vessel or cylinder headings. The key is whether the cylinder is identifiable by its design, dimensions, or markings as a fire extinguisher component.