Tomek, i think you are right that the top of seat is the most upper point. Your interpretation could be considered right according to the rule. Anyway it is hard to me to accept the two green “corners” with smaller radius. They could in your interpretaton be sharp corners as well. I think we should include them in the top of the seat back.
In words Tomeks interpretation could be something like this: “It has to be possible to draw half a circle with 2″ radius on the seat back that in the top of the circle intersect with the top of the seat back” (if i interpret it right).
I agree with Jeff.