Restriction on calling at non-customs places: conveyances may call only at notified customs stations, with narrow exceptions. Unloading of imports and loading of exports is allowed only at places notified as customs stations; the Principal Commissioner or Commissioner may approve specific places and define Customs area limits. A vessel or aircraft operator must not call or land at non customs places except with Board permission, or when compelled by accident, stress of weather or other unavoidable causes, which are treated as narrow exceptions.