IMO this is related to a much large problem which is trying to anchor types via the cognitive functions and preference order to begin with. It's so arbitrary its borderline retardation. CHALLENGE to all on this board that disagree with me on this: give me any thought, feeling or behavior and I can explain it using ANY combination of functions(you choose them for me), and i wont even have to stretch your imagination that much to do so. The cognitive processes theory is a "any shoe size fits" theory and thus explains nothing.. (rather, i should say it explains everything regardless of what way you want to explain it, which renders it entirely subjective and inconsistent)
MBTI theorists like evan says its like this to reduce the combinatorics problems you would run into if you let cognitive functions strengths/preference dictate how many types we would have. (which would be 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 = 40,000+ types). So what they do is settle on 16 and do their best to "wiggle" people into one of those types via cognitive functions. However, this does nothing to explain the "in betweeners" we see on this board and in the real world. If you ask a rigid MBTI theorist, they will tell you that a person with a Dominant Ni and auxillary Fi should not even exist according to definitions, nor should someone with dom Ne aux Ni, but they do and we have those people here present on this message board.
I suggest just try to ditch the whole notion of individual cognitive functions and look for your/other's COHESIVE type, and if you yourself as an XXXX type feel it convenient to explain something you do via a function then go ahead, but don't start with the cognitive functions firstly and definitely don't type others by the cog functions. And when you think about it, it makes a lot of sense. There is absolutely NOTHING in the description or definitions of Se that, say, allows you to understand how an ISTP (just as an example) sees the world and why they rebel so often against it. If you want to know how ISTPs go about the world, do not appeal to cognitive functions, just talk to/observe them, and you will understand ISTPs and be able to spot them. No appeal to dom/aux processes is needed and in fact I think it leads you astray.