The three xylene isomers — ortho, meta, and para — have distinct industrial applications and different market values, particularly p-xylene which commands a premium as a PTA feedstock. Separate tariff lines allow CBP and trade analysts to track isomer-specific trade flows. Importers must identify the isomer composition from the supplier's certificate of analysis and declare the correct line; mixed-isomer streams that do not meet a specific isomer threshold fall under the NESOI lines.

A certificate of analysis (CoA) from the producer showing isomer purity by weight percentage is the standard supporting document. If the product is a mixed xylene stream, the CoA should confirm that no single isomer predominates at a level that would require classification under a specific isomer line. Customs brokers may also request the production process description to confirm coal-tar versus petroleum origin.