Road Worthy: That is my understanding of the %/Volume/price(Platt) setting also. Unless the rest of the world decided to take the Mathematical term 'normalization' and change it. Meaning they establish the Fe Content, normalize it to the volume and from them on out they price the volume accordingly and therefore need only worry about weight/unit volume in the shipping.
HOWEVER, having said that, I will default, for the moment, to pesquero's decription since he has way more practical knowledge then I do in this feild.
Guess I had better get my but in gear and educate myself some more,
Steve