Originally Posted by
DaveReidUK
ICAO designators are assigned primarily on the basis of performance differences that are relevant to ATC, for example the extra 30,000 lbf of thrust on the B77W.
Thrust to weight would be more interesting, as it drives climb performance. Unfortunately ATC doesn't now the actual aircraft weight. Wingspan is important for stand planning as a stand for a 773 might not fit a 77W. I have heard of cases where this mixup almost caused the loss of a winglet.