@ Aussie: If you are on a cat A or C ticket, you can buy either an Economy class or Business class ticket.
If you buy the Economy class ticket, then that is the class you will travel.
If you buy a Business class ticket, you will travel Business class if there is space available, if there is no business class space you will travel Economy.
There is no facility to upgrade your ticket at the ticket counter if Economy is full, so you have to make the choice a day or two earlier which class you want to travel vs what risk of getting bumped off you are prepared to take.
It is hard to be succinct and precise in writing, yet in practise it is quite simple.