Synthetic staple fiber thread (550810) is made from polymer-based fibers such as polyester or nylon, while artificial staple fiber thread (550820) is made from cellulose-based fibers such as viscose rayon. The fiber origin — not the thread construction or packaging — drives the split between these two subheadings.

No. Heading 5508 explicitly covers sewing thread 'whether or not put up for retail sale,' so both industrial bulk cones and consumer retail spools fall here. Retail presentation may, however, affect the specific 10-digit tariff line and any applicable textile category.