Meaning of a "confirmed upgrade"

What is an “upgrade confirmed at time of reservation”?

I understand that there is some point about upgrades atlast moment, especially paid upgrades. Namely, as the departure approaches, the airline might find that there are seats left empty, in Coach as well as in Business. In this case, it may make sense to sell the remaining seats at a discount – various “last minute offers”.

But there would be problems finding new passengers for Business seats, even at discounted Business prices. So, a good market for the last minute offers of discounted Business seats might be the existing Coach passengers. That would be the paid upgrades shortly before departure or in the airport.

But what is the meaning of upgrades confirmed at the time of booking? How are they different from tickets booked as Business to begin with?