Vertical position of an aircraft at or below the Transition Altitude shall be expressed in terms of altitude relevant to the appropriate QNH setting. A climbing a/c will, at the TA, change its altimeter zetting to 1013.2mb.
The Transition Level is the lowest Flight Level (1013.2mb) available above the TA. At and above this level the vertical position shall be expresed interms of Flight Levels. A descending a/c will change its altimeter to the local QNH as it passes the TA.
There may not necessarily be any separation between the two but many ATC units will ensure that the lowest FL they allocate will be separated from the TA.
There... a pretty good guess without reference to any manuals!!!