I'm not sure if it is exactly the same system, but on the 747-400 THR REF & THR are explained as follows:-
THR REF Thrust is set to the "selected" limit on the EICAS. (This could be maximum power for takeoff or go-around, or whatever has been selected in the FMC for reduced power for takeoff).
THR Applies thrust necessary to maintain requested vertical speed (on a go-around the THR mode can provide thrust up to the [G/A] thrust limit, to achieve a 2000 FPM climb)
Hope this helps