Tyre shoulder?

[No_Style]

So I'm busy trying to make sense of this:
http://www.metzeler.com.au/shop/cate...etec-tyre-data

What the hell is the diameter (shoulder mm)?
I'm guessing it is the diameter of the tyre when measured between the points defined by where the sidewall meets the tread (i.e. the junction between the tread surface and the sidewall); is that about right, or am I way off?
Also, why do they use that as specification, why not use 190/60x17 or something mildly more conventional like that?

[Mstevo]

That would be the widest part of the tyre, as you've figured out

sportec4t.jpg