5510 - Yarn (other than sewing thread) of artificial staple fibers, not put up for retail sale:

Heading 5510 encompasses yarns, excluding sewing thread, composed of artificial staple fibers. These yarns are specifically not packaged for retail sale, meaning they are typically intended for industrial use, such as in manufacturing textiles. The defining characteristic is the presence of artificial staple fibers, which are short, discontinuous lengths of man-made materials, as opposed to continuous filament yarns.

This category is distinct from other headings within Section XI that might also deal with yarns or textile materials. For instance, it differentiates itself from yarns of natural fibers (e.g., Chapter 50-54) and from yarns of synthetic filament (which would fall under Chapter 54). Furthermore, the exclusion of sewing thread means that if the yarn's primary intended use is for stitching garments or other items, it would be classified elsewhere.

Within heading 5510, further subdivisions are made based on the composition of the yarn. For example, subheadings differentiate between yarns where artificial staple fibers constitute 85% or more of the weight, and those where other materials are mixed in. The classification then refines these mixtures based on whether the predominant admixture is wool, fine animal hair, or cotton, or if the yarn falls into a general "other" category. This detailed breakdown allows for precise tariff application based on the specific blend and material composition of the artificial staple fiber yarn.