Some background first. FL600 is 60,000 feet, the altitude at which Air Traffic Control is no longer in charge, if you can climb above that point you're on your own. A modern airliner (777 for example) cruises at FL450 and isn't super happy to fly above that.
- SR71: QQ requesting clearance to flight level 600
- ATC: QQ, climb and maintain flight level 600... if you can get there.
- SR71: Roger, descending to flight level 600 from 850.
- ATC: Uh... roger