Just going through the learning curve with my 750iL.

To the immediate left of the radio, there's a hole, something missing from the dash. I looked in the owner's manual and it appears that there should be a vertical dial, similar to the one that regulates the dashboard light intensity. I see it pictured in the manual, but there is no description as to what it does. So, tell me what does this dial do? Thanks in advance for the help.