I might be wrong but I assumed it was just a 'button' to indicate the centre of the button panel.Its not a button, its a sensor for either temperature or light. If you look around the outside edge of the central display there are a couple more at about 4 o'clock and 7 o'clock
Oops! I thought you were asking about the Item where your thumb is.It displays the temperature you have set on either side with climate control.