*warning there is no spell check on this computer so please try and stay with me
It comes down to the brands a bit, there isnt any standard rating so companies can claim any rating they like really. There is alot more into it than i have time to give you but a brief 101
To make things a bit more confusing there are a few different skimmer types air driven, venturi, needle wheel, mesh wheel, grid wheel and so on.... some are alot more agressive than others, the generally pricipal is the finer the air bubbles it produces the better it is as the more contact area is present.
The commline serries of the Tunze is classed as very gentle and they use a what they call hydrofoamer, which from what I gather is their version of ventrui, which is a tried and true method but very gentle and leaves alot of the protein (along with some good things as well) in the water coloum.
Then there comes the design of the body of the skimmer which includes its shape, the way the water is injected, the height, if there is anti tubulance plates, where the outlet is positioned the neck diameter and all the ratio of air/water/body size as well.....
Depending on the method you generally want a round body so water can flow smoothly and evenly, square bodies like the tunze comline arnt the ideal shape, you generally want a fairly tall skimmer to increase to contact time the air is exposed to water, anti turbulance plates give a smoother and more consistant internal flow with increased contact time (if done correctly) then you have things like cone skimmer etc which in theory help alot but i havnt really seen any difference in them over straight sided skimmers.
And once you think you have all this figured out and use the above like a checklist, skimmer can have all of the good features but just dont work well in practice. List a skimz sm121 on paper looks better than a KZ small but in reallity only performs 10% as well. So it also comes down to trusted names and designs and I hate to say it but you largely get what you pay for.
Despite all its short comings the Tunze comline actaully manages to perform well, there are alot better performing skimmers out there, but that isnt to say its a bad choice.