Bosch Fridge Freezer Classixx . I have a Bosch Classixx Fridge Freezer which has a button in the

Question

I have a Bosch Classixx Fridge Freezer which has a button in the fridge section located next to the temperature wheel which goes from 0 to 5. If you switch the button, a red mark appears.... I am not sure what function this button provides.... is it a fast freeze button or does it have something to do with keeping the fridge freezer at the correct temperature when the outside temperature changes? Thank you.