Not sure what the question is, but...
It's transition altitude on the way up, usually 3000' in uncontrolled airspace in the UK, but often higher in CTZs. It is a fixed, published altitude, climbing through which you set 1013.
Coming down it's transition level you're interested in; this is the Flight Level, below which you set QNH and fly by altitudes once again rather than FLs. The transition level is given by ATC or the ATIS and varies according to the QNH, so that ac on FLs and ac flying altitudes don't bump into each other, when the QNH is below 1013.
The airspace in between the TL and the TA is the Transition Layer.
In practice, in CAS set 1013 on the main altimeter when cleared up to a FL. Outside CAS set 1013 when passing the TA. Coming down, set QNH when cleared to an altitude.
Hope this helps, if you needed it <img src="smile.gif" border="0">
[ 18 February 2002: Message edited by: Brakes...beer ]</p>