Most true slicks for road course use are designated as such. The tire diameter is crucial in the formula to find the perfect gearing for a specific course. I suppose it's easier to change tire diameter over gear ratio.
The middle number, e.g. 650, corresponds to the tire diameter in millimeter which, in this case corresponds to 25.6 inches. This is easier to use when a change in diameter is required but the width kept the same. Of course the effective diameter changes as the tire wears during the course of the race. The first number still corresponds to the tread width and the third the wheel diameter.
These tires can be ordered in 5 mm increment for both width and height; a much more accurate method than the 5% increment in aspect ratio and 10 mm in width for street tires.
I've run 645 mm upfront and I prefer 650 instead in order to maintain a slightly taller ground clearance. Maybe it's the wrong approach since the purpose is to keep the Z06 from scrapping the air deflector under full braking. My previous Z06 did the same with much smaller front brakes.