The distinction is chemical origin. Artificial staple fibers — viscose rayon, modal, lyocell, cupro — are derived from cellulose, while synthetic staple fibers — polyester, nylon, acrylic — are petroleum-based polymers. A yarn's fiber identity, not its appearance or end use, determines which heading applies.

If the yarn contains less than 85% modal (an artificial staple fiber) and is mixed mainly with cotton, it falls under subheading 551030. If the modal content is 85% or more, it would be classified under 551011 (single) or 551012 (multiple or cabled), depending on yarn construction.