Enclosed are a few photos of one of a pair of Chinese Export dishes that I got recently, dating to the Kangxi or the Yongzheng period. They both have a raised boss in the center.
Wine tasters have a similar center that helps in judging the color of the wine.
It has been suggested that dishes of this kind was to be the under dishes for ewers with the raised center there to fit the under side of the ewer, but I have never seen any dishes like these with even the tiniest wear to indicate they have ever been used like that. If someone really knows this, I would be happy if someone would send me an email about it. This information is given as an example of private conversation only and is not intended to be used as a promotion of any individual piece. All opinions are the authors and are given as such with all hazards of judging anything from a photo.