8r, 6r, etc is an arbitrary label invented by specialized to indicate "this carbon is higher quality than the other stuff". 8r on one bike could be made completely differently than the other one.

In the Tarmac I rode the 6r, 8r, and 10r. The 6r would rattle your wrists on any road imperfection. I knew I would never like that. The 8r would only rattle your on bigger things on the road. It seems to absord or dissapate the hit for small stuff, but big stuff still got through. The 10r soaked it all up. Unfortunately, they put super stiff wheels on the 10r so while my wrists felt a lot better, but butt and the rest of my body got more of a beating. :-/

I don't know the exact specifics of how it worked, I just know the end result from having done a 15 miles test ride with each bike (yes I filled up the tires to the same pressure before I went out).