A hosho-sen was a ship carrying not only a shuinjo (license for foreign trade) issued by the Seii Taishogun (literally, "great general who subdues the barbarians") but also a hosho (permit) written by roju (members of shogun's council of elders).
奉書船（ほうしょせん）とは、征夷大将軍が発給した朱印状に加えて、老中の書いた奉書という許可証をもった船のこと。