The metric number (Nm) reflects the fineness of the yarn and directly determines which subheading applies. Finer yarns (higher Nm) are used in lighter, higher-thread-count fabrics and command different duty treatment than coarser yarns. Importers should obtain mill test certificates or yarn count test reports to substantiate the declared Nm, as misclassification by count is a common audit finding.

For multiple or cabled yarns, the Nm used for classification is measured on the individual single yarn before plying, not on the finished multiple yarn. This is consistent with the HTS chapter notes and means that a two-ply yarn made from two 40 Nm singles is classified in the 14–43 Nm subheading range, not at the combined count.