"Yun Bei" is what we call this tea serving jug in Taiwan. Yun means "matching" and "bei" means cup or bowl, so in English it could be called "tea matching vessel."
 
Its purpose is to ensure that all diners drink the same tea, since by serving from the teapot, the Yun Bei will balance the flavours and aromas of the top of the teapot with those of the bottom. You can try pouring directly from the teapot into two cups, one after the other, and you will notice differences in the tea in each one. Hence the reason for using this accessory when preparing Gongfu style tea.
 
This Yun Bei holds 380 cc and is perfect for large teapots of 150 cc or more designed for Gonfu sessions of 4 or more diners. You can find our teapots and Gaiwans of that size on this link.