Subheadings 550911 through 550942 apply when the yarn contains 85% or more by weight of the named synthetic fiber (nylon, polyester, acrylic, or modacrylic). When the content falls below 85%, the yarn is classified in the blend subheadings (550951–550999), which are further split by the predominant secondary fiber — cotton, wool or fine animal hair, artificial staple fibers, or other.

Because the polyester content is under 85% and the yarn is mixed mainly with cotton, it falls under subheading 550953 (polyester staple fiber yarn mixed mainly with cotton, under 85% polyester, not for retail sale). If the secondary fiber were wool or artificial staple fibers instead, a different blend subheading would apply.