Duck is technically a plain-weave construction, while twill has a diagonal interlacing pattern — they are structurally distinct. If a fabric's weave is genuinely ambiguous, a textile laboratory analysis identifying the interlacing structure is the most defensible basis for classification. Duck-weave fabrics would fall under 5211390060, while twill constructions fall under 5211390040, so the weave determination has a direct tariff-line consequence.

When sourcing from multiple origins, importers should maintain separate mill certifications and fiber-content test reports for each origin, as CBP may audit classification consistency across shipments. For fabrics from China, Section 301 tariff applicability must be confirmed. For Mexico-origin fabrics, USMCA origin certification should be obtained if preferential treatment is claimed. Keeping origin-specific documentation organized by shipment is essential for any post-entry audit.