E-Tune is a trim level...
Much like Toyota's GL, GLS, etc...
Or Fords, XT, S, ect (or like the new G series & non G series)
The reason why E-Tune are only on the "D" revision models is because they were the last of the model range and Subaru had to do something to stay on a MARKETING par with Mitsubishi, Toyota, Lexus, etc... : : : : :
Car Manufacturer & Dealers Modes operandi 101: "Dream up some cool marketing gimmick, and use it to show case old model against competitor's new model untill our new model is availble..."
I use the above on a daily basis at work because my product is exactly the same as my closest competitor, hence I need a clever sales/marketing trick to get the sale and deny my competitor the sale
So all say: TRIM LEVEL...