Heading 1518 applies when the oil has undergone a chemical reaction that changes its molecular structure—such as oxidative polymerization (stand oil), sulfurization, or dehydration. Physical processes like filtering, bleaching, deodorizing, or fractionation do not trigger reclassification. The key test is whether the fatty-acid ester bonds or carbon chain have been chemically altered, not merely whether the oil's physical properties have changed.

Inedible mixtures classified here are generally not subject to FDA food-safety review, but certain chemically modified oils used as industrial chemicals may require EPA Toxic Substances Control Act (TSCA) compliance if they are new chemical substances. Importers should verify whether the specific product is listed on the TSCA Chemical Substance Inventory before entry.