The countries involved as suzerain nations under China offered tribute to the Chinese emperor, politely referred to as a kind of "gift" to China. In Japanese this tribute was called either "Shinkou" or "Chokou."
In the Ryukyu Islands, the Chuzan King Satto was the first to participate in this type of relationship with the Chinese, followed by the competing kings from the Nanzan and Hokuzan kingdoms. When the islands were unified as the Ryukyu Kingdom under Sho Hashi, the monopoly they enjoyed under this tributary trade relationship enabled rapid development in economic power for the kingdom.
The groups of vassals dispatched to China for trade and tribute numbered 300 people and ships were dispatched about once every two years. The vassals dispatched to China were permitted an audience with the emperor during which they presented letters from the Ryukyuan king and tribute goods. In exchange they received letters from the emperor to the Ryukyuan king and many high quality Chinese products.
The tribute offered to the emperor consisted of Okinawan goods including horses, sulfur, shell products, and Bashofu banana cloth fabric as well as crafts goods from Japan and rare products from Southeast Asia. The tribute brought in from the Ryukyus was of great military value to the Chinese and so the Ryukyu Kingdom was regarded as an important supply country. Moreover. because of the enthusiasm with which they engaged in the tribute and trading system, they were received warmly by their Chinese hosts compared with other countries. Gradually, the Ryukyu Kingdom was designated by China as the intermediary trader in East Asia and Southeast Asia.

Shipping route of the tribute and trading ships between the Ryukyus and China.

Seal from the Emperor of China Presented to the King of the Ryukyu Kingdom