ICAO permits (or did permit) 4 types of TL, viz
1. Lowest FL above TA, with separation between them (variable with QNH).
2. Lowest FL above TA, without separation between them (variable with QNH).
3. and 4. As 1 and 2, but a fixed, notified TL. This is possible in parts of the world with a stable pressure pattern, though I trust it's reviewed from time to time!
Hence the slightly lose definition of "available for use above the TA" - you pays your money and takes your choice.
[This message has been edited by 2 sheds (edited 18 November 2000).]