Stak has many more functions than there are keys on a mobile phone, so it has three shiftstates (Stak16 has four), toggled with the "#" key.
Functions are not executed when a key is pressed, instead the function name is shown in the upper right corner of the screen, and if it is released within 0.6 seconds it is executed. If the key is held down the operation is cancelled.
If the shift key ("#") is held down it is cancelled just like the other keys, and a map of the keyboard is shown on the screen. The map goes away when a funtion is executed, or if the shift key is held down again. This feature is avaliable in all modes.
Stak16 has a user-definable keyboard. The CAT function goes to a list of possible operations, which can either be executed directly or assigned to a key. "User mode" in the MODES view switches between the user defined keyboard and the default layout.