1x = slow key repeat rate, calculator icon starts Windows calculator.
2x = faster key repeat rate, "PC" icon starts Windows Explorer (I think, I am on my Mac at the moment).
3x = even faster key repeat rate, envelope icon opens default email application.
4x = fastest key repeat rate, house icon opens default web browser.
I might have the functions of 2x and 4x swapped, as none of them do anything on the Mac.