The buttons can be used in general to take screenshots and to open the xiaomi notes app. When using the xiaomi notes app and some other note taking apps (not all) you can also use the buttons to switch between pen types and the eraser function as well as the pen color options. There is however not a designated eraser button. Also, with some apps to read pdf files it's also possible to change pages by using the buttons.

This is possible according to the operating instructions.