Filament thread is made from continuous strands of synthetic fiber (such as polyester or nylon filament), producing a smooth, lustrous, and strong thread with minimal hairiness. Staple-fiber thread is spun from short fiber lengths, resulting in a softer, slightly hairier surface more similar to cotton thread. The distinction is both a physical characteristic and a classification criterion — filament and staple synthetic threads fall under separate SITC headings, so the thread's construction method must be confirmed with the supplier.

Synthetic sewing thread from certain origins has historically been subject to antidumping duty investigations and orders. Additionally, synthetic filament products from specific countries may be subject to Section 301 tariffs. Importers should verify the current status of any applicable orders with CBP's ADCVD database and consult a licensed customs broker to assess total duty exposure before finalizing sourcing decisions.