Manu gives a book answer but just in case it ever happens to a pilot reading this, a controller can ask the pilot to set any altimeter setting and fly at an appropraite indicated level. The aim of the game is to separate aircraft (either from each other or from terrain).
It may not be viewed as good practice but it does happen. This may mean that an aircraft is asked to fly at an altitude above TA. Theoretically, an aircraft below TA could be assigned a FL but whilst altitudes above TA are not uncommon, I don't think I've ever seen the latter.
There are many other factors to be considered but it can be done - inside or outside CAS!