The distinction is based on fiber diameter and animal species. Fine hair generally comes from specialty animals such as cashmere goats, Angora goats, camels, alpacas, and vicuñas, and typically measures below a defined micron threshold. Coarse hair — from horses, cattle, pigs, and similar animals — falls under Chapter 51's coarse category. When the species or diameter is ambiguous, a licensed customs broker or CBP binding ruling can resolve the classification.

Coarse animal hair is used in brush and broom manufacturing, traditional upholstery padding, felt production, and specialty filtration materials. Horsehair in particular is valued in musical instrument bow-making and high-end furniture. These niche industrial demands, rather than apparel, define the import market for this heading.