5508 - Sewing thread of man-made staple fibers, whether or not put up for retail sale:

Heading 5508 specifically classifies sewing thread manufactured from man-made staple fibers. This includes threads that are ready for retail sale, such as those found in small spools or kits. The key defining characteristic is the material: fibers that are short, discontinuous lengths of synthetic or artificial origin, spun together to form a continuous thread.

This heading is distinct from other headings within Section XI, Chapter 55, which deal with the raw man-made staple fibers themselves or fabrics made from them. Unlike headings for yarns (which can be continuous filament or spun staple) or fabrics, 5508 is solely focused on the end-use product of sewing thread derived from staple fibers. It also differs from sewing thread made from natural fibers, which would be classified under different headings.

The subcategories within 5508 differentiate based on the specific type of man-made staple fiber used. 5508.10 covers sewing thread made from synthetic staple fibers, such as polyester, nylon, or acrylic. Conversely, 5508.20 encompasses sewing thread composed of artificial staple fibers, which are derived from regenerated cellulose or other chemically processed natural polymers, like viscose or modal. This distinction is crucial for accurate duty assessment.