Share 'A/C control mysterious "Third Eye"'

Ok, I admit it. I am stumped. If you look at the "Air Distribution Control Knob" (<<---- anyone want to bet or confirm that in German that is all one word? hehehehe), there is, at the bottom, a little "eye" or lense looking thingy. I have pored over the owners manual, but I can find no explanation for this item. I initially thought it would be some sort of light that would let me know when…