Customs supervision governs loading and unloading of imported and export goods, subject to limited permission-based exceptions. Imported goods must not be unloaded from a conveyance, and export goods must not be loaded on a conveyance, except under the supervision of the proper customs officer. The provision establishes customs supervision as the default rule for loading and unloading of goods carried by conveyances, while preserving limited administrative flexibility through permission mechanisms. The Board may grant general permission by notification, and the proper officer may grant special permission in a particular case for unloading or loading without supervision.