The thrust reference is the actual thrust calculated and used for takeoff.
The Assumed Temperature Derate, if selected, is used to calculate the takeoff thrust. Perhaps the pilot selects an assumed temperate of 35C (say it's 15C outside). Then the Thrust Management Computing Function (TMCF) in the 777 calculates what thrust that is. For example, an assumed temperature selection of 35C may result in 99.9 N1. Full underated power may have been 103 N1.
So in this example, the THR
REF value would be the result of whatever Assumed Temperature Derate was selected. You can also do un-derated full takeoff power, or another method called Fixed Derates (10% and 20% reductions, usually).
In other words, the THR REF value is the result of what assumed temperature was selected.
There are several ways you can select what the THR
REF value will be for takeoff - either manually or the computer calculates it for you:
1) Full un-derated takeoff power
2) Select an Assumed Temperature Derate
3) Select a Fixed Derate (TO1 or TO2)
4) Select a combination of an Assumed Temperature Derate on top of a Fixed Derate (say TO1 + an assumed temp)
5) Select a Takeoff Bump on those airplanes so equipped (this is a special higher than normal thrust, opposite of a derate)
6) Manually select a THR
REF in the CDU
. For example, you can just type in the takeoff thrust, say 99.9 rather than have the TMCF calculate it for you
7) Uplink a THR
REF to the TMCF by datalink (a.k.a. ACARS).
Not all airplanes can do all of these. If you do #6 or #7, presumably there's a dispatcher figuring all this stuff out for you. Pilots don't just make up a takeoff THR REF value.
You can derate climb thrust too, but only fixed climb derates (CLB1 and CLB2).
Confused yet?[Edited 2012-06-06 12:35:54]
[Edited 2012-06-06 12:40:56]