Import General Manifest requirement restricts unloading unless manifest filed, ensuring customs supervision of cargo movements. Carriers must file a prescribed Import General Manifest or import report before unloading and an Export General Manifest or export report before departure; unloading or loading not covered by those filings is prohibited or restricted. Preventive Officers supervise cargo handling to verify completion of customs formalities, payment obligations where leviable, and proper authorization for export, with manifests serving as proof of shipment.