It's the snow mode. From my manual:
Snow programme
In position D the snow programme offers smooth driving adapted to slippery roads, so as to improve traction and stability. Starting is in second or third gear depending on the engine and changes into lower gears are performed at low engine speeds.
In positions 3, 2, or L the gearbox operates as a manual one: in position 3, the car is started and driven in third gear only. In position 2 or L, starting and driving are in second gear only
* button on auto box
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Re: * button on auto box
<steve_earwig> I think this forum is more about keeping our cars going with minimal outlay than giving our cars more reason to go bang