The key is whether the yarn is 'put up for retail sale' — meaning packaged in quantities and presentations intended for direct sale to consumers, such as labeled balls or skeins. Yarn on industrial cones or in bulk packages for manufacturing use belongs in 5509 (synthetic) or 5510 (artificial), even if the fiber content is identical.

If the yarn contains less than 85% synthetic staple fibers (acrylic in this case) and is put up for retail sale, it falls under subheading 551120, which covers retail-pack synthetic staple fiber yarn with under 85% synthetic fiber content. The wool component does not shift it to a wool heading as long as synthetic staple fiber is the predominant fiber.