5509.51 - Mixed mainly or solely with artificial staple fibers:

This HTS category, 5509.51, encompasses yarns, not intended for retail sale, that are spun from synthetic staple fibers, with a primary composition of polyester staple fibers. Crucially, these yarns are characterized by being mixed mainly or solely with artificial staple fibers. This designation distinguishes them from yarns composed primarily of other materials, even if they contain a small proportion of artificial staple fibers. The "mainly or solely" aspect is key for accurate classification.

Within this level, category 5509.51 differentiates itself from its siblings by the specific nature of its admixture. Unlike 5509.52, which is mixed with wool or fine animal hair, or 5509.53, mixed with cotton, this category is exclusively for polyester staple fiber yarns blended predominantly with *artificial* staple fibers. Category 5509.59 serves as a residual category for polyester staple fiber yarns mixed with other unspecified materials.

The subdivisions within 5509.51 are based on the construction of the yarn. Subcategory 5509.51.30.00 covers single yarns, which are formed by twisting a single strand of fibers. In contrast, subcategory 5509.51.60.00 pertains to multiple (folded) or cabled yarn. This distinction is important as it reflects different manufacturing processes and potential end-uses of the yarn, impacting its physical properties and, consequently, its classification.