A yarn containing 85% or more wool by weight is classified under 510610 regardless of what other fibers are present. If wool content falls below 85% — for example, in a wool-acrylic or wool-nylon blend — the yarn moves to 510620. The weight percentage is measured on the total fiber content of the yarn, and the determination should be supported by fiber analysis documentation.

Carded yarn is spun on the woolen system from shorter, randomly oriented fibers, producing a bulkier, softer, and less uniform yarn suited for tweeds, blankets, and felt. Combed yarn (heading 5107) is spun on the worsted system from longer, parallel-aligned fibers, yielding a smoother, stronger yarn used in suiting and fine knitwear. The spinning system used — not the fiber length alone — determines which heading applies.