I have to use the panasonic UX180 at a studio once in a while and an trying to learn how to use it without having access to it most of the time.

According to the user manual, pressing the white balance button cycle between Ach, Bch, and PRST

My understanding is Ach and Bch are custom white balance settings set by the user so he can easily recall one of A or B settings.

So what is the PRST setting? Is it like a third user customizable white balance setting? If so, why isn't it called Cch?

PRST stands for Preset. In the menu white balance settings you can manually set PRST to what ever you want. Usually users set PRST for 3200K (indoors) or 5600K (outdoors).