In practice: the Harmonized System is maintained by the World Customs Organization. The code decides the duty rate, which permits or conformity certificates are needed, and whether the goods are restricted. The same code and a matching description should appear on the commercial invoice, packing list, certificate of origin and declaration; mismatches are a common reason for rejections, especially in Iraq's ASYCUDA system.

Example: an importer declares LED lamps under a general “electrical parts” heading. Customs reclassifies them to the specific heading, applies a different duty rate and asks for a conformity certificate, and the container waits in the port while the documents are corrected.