What is Jiugongge?

Draw a square horizontally and vertically in the middle, and divide it into nine small squares with equal area, commonly known as Jiugongge.

Application: As far as I know, it should be a subtle footwork in Tai Ji Chuan China Boxing, that is to say, the place where people stand is in the middle of Jiugongge, and how to move footwork is exquisite.