The threshold is measured by weight of all textile fibers in the fabric. If flax fibers constitute 85% or more of total fiber weight, the fabric falls under 530911 or 530919 depending on finish. Below that threshold, it classifies under 530921 or 530929. Accurate fiber content testing and mill certificates are essential for correct classification.

Dyeing, printing, and any finishing process that alters the fabric beyond simple bleaching — such as mercerizing, coating, or piece-dyeing — places the fabric in the 'other' subheading (530919 or 530929). Unbleached greige goods and fabrics that have only been bleached white remain in the unbleached/bleached subheadings.