Rather simple, really. For now, it's only grain tank capacity and engine horsepower.
Personally, I think manufacturers need to get on the ball with other standards, such as cylinder width, rotor size, sieve area, walker area [where applicable] and anything else they may want to add. The former PAMI was to combines what the Nebraska Tests were to tractors. More of the PAMI standards could be applied; otherwise, we could throw a 475 Hp engine and a 400 bu bin on a John Deere 7700 and make a Class XI out of it.
Thanks for your reply but it doesn't make sense to me. I understand and believe what you say but a 06 r-75 has 330 bushel bin and 300 hp and a class 7, a 09 r-66 has a 300 bushel bin tank and 300 hp and is a class 6. Doesn't all figure.