The distinction turns on when color is applied. In 551321, the woven fabric is dyed as a whole piece after weaving, resulting in a uniform color. In 551331, individual yarns are dyed before weaving, creating woven-in patterns such as stripes, plaids, or checks. CBP evaluates this based on the fabric's construction, and a cross-section examination can confirm whether color penetration is uniform (piece-dyed) or varies by yarn (yarn-dyed).

Jacquard-woven fabrics using differently colored yarns may fall here if they are plain weave in overall construction, but complex jacquard structures are more likely to be classified under the 'other weave' headings. The weave structure — not just the yarn-dyeing method — determines the correct 10-digit line, so importers should confirm the weave type with the mill before filing.