Synthetic filaments are polymer-based fibers such as polyester, nylon, or polypropylene, derived from petrochemical feedstocks. Artificial filaments are regenerated cellulose-based fibers such as viscose rayon or cupro. The fiber chemistry determines the subheading, not the thread's end use or packaging format.

Heading 5401 explicitly covers sewing thread 'whether or not put up for retail sale,' so packaging format does not change the classification. The defining characteristic is that the product is sewing thread — typically finished with lubricants or surface treatments for needle passage — rather than a general-purpose filament yarn.