Metric number (Nm) is a yarn count system where higher numbers indicate finer yarns. The 100 Nm threshold in 6006211000 separates fine-yarn circular knit fabrics — typically used in lightweight, high-quality jersey — from coarser constructions. Importers must obtain yarn count data from the mill to correctly assign the tariff line, as misclassification between the fine and coarse lines can affect duty rates.

Single knit (or single jersey) fabrics are produced on machines with one set of needles, creating a fabric with a distinct face and back. Double knit, interlock, and rib constructions use two needle beds and produce more stable, reversible fabrics. The distinction matters because 6006219020 specifically captures single-knit constructions, while 6006219080 covers all other non-circular or non-single-knit variants not elsewhere specified.