Woven fabrics containing 85% or more synthetic staple fiber by weight are classified under heading 5512, while fabrics below that threshold mixed principally with cotton fall under 5513 (under 170 g/m²) or 5514 (over 170 g/m²), and other mixed fabrics under 5515. The fiber content percentage by weight — not by count or visual inspection — is the controlling criterion, and lab testing is often required to confirm it.

Filament tow consists of large bundles of continuous synthetic or artificial filaments that have not yet been cut to staple length; it is classified under 5501 (synthetic) or 5502 (artificial). Once those filaments are cut or broken into discrete short lengths, they become staple fibers under 5503 or 5504. Tow is an intermediate product destined for further processing into staple, and importing tow versus staple can carry different duty rates.