Area class is a super class for every other MUI class except windows and applications. It holds information about an objects current position, size and weight and manages frames, fonts and backgrounds.
Additionally, area class handles the user input. By setting an objects MUIA_InputMode, you can make it behave like a button or like a toggle gadget. That's why MUI doesn't offer an extra button class. A button is simply a text object with a raised frame and a relverify input mode. Since especially group class is a subclass of area, you can create rather complex buttons consisting of many other display elements.