Hi MB2002,
The THR DES mode is a feature of the A330/340. Basically THR DES is the sum of THR IDLE added to (sorry I am on a weird keyboard can't find the plus sign
) the IDLE factor. The IDLE factor is used to adjust the IDLE thrust level and is a percentage of the basic idle thrust. You can insert it and modify it from IDLE/PERF page. It is not to be confused with the PERF factor that is used for fuel predictions to take into consideration the engine wear. If you have a positive IDLE factor the airplane will start the descent earlier and when reaching the profile will go in THR DES. That extra amount of thrust compared to idle thrust will give you some flexibility during descent if conditions change such as wind speed and direction, the need of anti ice, etc.. basically all it does is to make you descend with some more thrust that in idle thrust. If you get high on profile the airplane will accelerate like on the A320 and will reduce the thrust to THR IDLE
On the A320 you have the same function but it doesn't show you the THR DES FMA, it's all in THR IDLE.
Cheers !