The second dot is for further distances, ie. what ever it may happen to be with your rifle/load. As you can see, it is slightly smaller, as to not cover as much out at 300m for example.
I switched over to the K18i recently from the Z8i. I do not especially like the second illuminated dot.
But overall it is a great scope. I was using the K16i for a number of years prior to the Z8i, so not my first time around with Kahles.