The distinction is the number of chlorine substituents on the benzene ring: pentachlorobenzene has five chlorine atoms and classifies under 2903930000, while hexachlorobenzene has six and classifies under 2903920000. Both are Stockholm Convention-listed POPs, but they are separate chemical entities with different CAS numbers, so the supplier's certificate of analysis confirming the exact compound is essential for correct classification.

Legitimate commercial applications are extremely limited given international restrictions. Residual trade is largely confined to analytical reference standards, research quantities, or use as a process chemical where no viable substitute exists and a specific regulatory exemption applies. Any importer should obtain EPA guidance under TSCA before proceeding.