The key criterion is acrylonitrile content: acrylic fibers contain 85% or more acrylonitrile by weight, while modacrylic fibers fall between 35% and 85%. Importers should confirm fiber composition from the manufacturer's technical data sheet before declaring the classification, as misclassification between the two can affect applicable duty rates and any trade-remedy exposure.

Certain origins of acrylic staple fiber have historically been subject to antidumping duty orders. Importers should check the current scope of any active orders with the US International Trade Commission and USDOC before shipment, as scope rulings can affect whether a specific fiber specification or country of origin falls within a covered order.