A Diamond Force is for paid rooms. And it usually means a paid room at rack rate.

It used to be if you wanted a reward room and no reward rooms were available, the Diamond desk would often contact the hotel to see if they would open up a reward room. Now with no "blackout dates" for reward rooms that is not needed. But if the hotel is sold out, then no rooms are available for rewards.

A Diamond force can be made but at whatever current rate the hotel wants to charge (ie rack rate).