Phil,
T6 is the turbine inlet temperature - if it is too high you can easily cook the HP turbine into an unuseable mess!
For interest, the general list is (IIRC):
0 - Ambient air.
1 - Air at compressor inlet.
2 - Air at exit of LP compressor.
3 - Air at exit of HP compressor.
5 - Combusting gases in chamber.
6 - Gas at HP turbine inlet.
7 - Gas at LP turbine inlet.
8 - Exhaust Gas.
Where Tn refers to the temperature and Pn refers to the pressure.
Don't be confused with N1 and N2, which are also refered to as NL and NH - the percentage rotation speed of the LP/HP spools.
T6NL caption, anybody?
Blunty (ready to be corrected!)