Check the legends on the numeric keypad:
Eight directions, "Left Button", "Both Btns", "Right Button", "Button Lock", "Button Release", "Double Click" and "Click".
Obviously, you would be able to use the numeric keypad as a serial mouse, in case you did not have any plugged in.
I suppose that the key with the mouse legend is for toggling between mouse-keys and normal keys on the numeric keypad.
BTW, I am pretty sure I have seen this keyboard before somewhere, either here or on Deskthority.