Classification of blended yarn: presence of natural fibres shifts tariff classification from synthetic-only subheading to general blended-yarn subheading. Classification depends on fibre composition: yarn with acrylic predominance that contains only other synthetic staple fibres (including waste derived fibres) is classifiable under 5504.31; if the yarn also contains natural fibres, it falls outside 5504.31 and is classifiable under 5504.39.