The Cha Hai (茶海) or also Gong Dao Bei (公道杯) is a much-used companion in the preparation of Chinese teas.
Small teapots and even smaller tea bowls are quite common in China. In the course of portining, it is certainly practical to serve the tea directly from the teapot, but this has the disadvantage that the taste of the infusion in each individual cup is different. The reason for this is that the tea leaves can continue to steep in the pot during serving, which means that the last portioned cups can have a particularly intense flavour.
The solution is to use this small glass decanter. When the tea is poured from the teapot into the decanter, the result is a uniform tea infusion, allowing an identical taste for everyone drinking. Incidentally, the literal meaning of Gong Dao Bei is derived from this functionality: "Fairness Cup".