Synthetic filament yarns (heading 5402) are made from polymers such as polyester, nylon, or polypropylene — materials that do not exist in nature. Artificial filament yarns (heading 5403) are made from regenerated natural polymers such as viscose rayon, modal, or cupro. The chemical origin of the polymer, not the yarn's appearance or end use, determines which heading applies.

Synthetic monofilaments of 67 decitex or more with a cross-section not exceeding 1 mm, or strips not exceeding 5 mm in width, fall under heading 5404 (synthetic) or 5405 (artificial), separate from multifilament yarn. Below 67 decitex, a monofilament is treated as yarn under 5402 or 5403. The decitex threshold and cross-section measurement are the controlling classification criteria.