Plastic monofilaments with a cross-section of 1 mm or less are classified as man-made filaments in the textile chapters, not under heading 3916. This means that a fishing line or industrial filament just above or below that threshold can fall into entirely different chapters with different duty rates and regulatory treatment. Measurement of the actual cross-sectional dimension — not the nominal product description — governs classification.

Surface working includes operations such as polishing, painting, or coating that do not change the profile's cross-sectional shape or convert it into a finished article. Cutting to length is generally acceptable, but machining, drilling, or assembling the profile into a component typically takes it out of heading 3916 and into a more specific heading for the finished article or part. CBP has issued rulings on specific profiles, and consulting those or requesting a binding ruling is advisable for complex shapes.