In order to distinguish legitimate traders from wako (Japanese pirates), a kangofu (certificate to show that the ship is lawfully registered by Ming) was issued in return for a tribute, and restrictions such as a limit of ten years per dynasty were imposed.
倭寇と区別するため勘合符を発行して相手を承認する朝貢形態で行われ、十年一朝など制限がされていた。