Customs control and clearance require mandatory manifests, valuation compliance and supervised cargo processing to prevent smuggling. The Manual explains that duties are levied under tariff classifications and that ad valorem duties require valuation under an international rules-based system; it requires carriers to file Import and Export General Manifests within prescribed timeframes and prohibits unloading, loading or departure without manifests and port clearance. Designation of ports, appointment of custodians, supervised storage, appraising staff scrutiny, selective physical examination, and use of electronic systems are presented as the core mechanisms for assessing duty liability, preventing smuggling, and regulating release of goods.