It's a small distinction; it was the first instance of socionics admitting that people (And by that I mean almost everyone.) simply didn't fit the behavioural patterns set out. The distinction is Process (irrational focus) and Result (rational focus). For example a IEI Ni will have a slight preference for methodology over result, while an IEI Fe will will have a slight preference for finishing/end result. Think of it as the difference between someone using MBTI saying I'm 100% P versus someone saying they're 60% P.
While this is an interesting PoV regarding the subtypes, I'm not sure I agree. I don't see how methodology correlates to irratonal functions while end results correlate to rational ones; if anything focus "end result" seems like how

appraoches ideas while focus on "methods" is more like

. Introverted thinking will look at something and think "this is what the conclusions are, this is what is being said" whereas extroverted thinking will look at it and say something like"this is how they arrived at these conslusions", focused just as much (if not more) on the means than the end. Another reason I disagree is because the words "process/result" refer to a Reinin dichotomy.
But this is getting off-topic which is the subtypes, I feel that if we are talking two subtypes theory, and we take an IEI, which is defined by two functions

conservative and

creative. The dominant function is a pool of information that I call "conservative" because it has its opinion on this apsect of reality which rarely ever changes. The creative function is constantly modifying its opinion about this aspect of reality, seeing it not as an objective reality, but something to modify and amend as a personal skill. So an IEI can prefer either the concervative

process which holds to its values, or the creative

process which values change and diversity in this aspect. I think the subtype which prefers the dominant/conservative function appears more infexible, whears the other subtype appears more open-minded. The first conservative subtype is thus a more "classic/stereotypical" model of their type in terms of what they value, an IEI-Ni will value imagination and stick with the fantasies of their childhood till the end(the stereotypical IEI) , whears the creative subtype, while it will have some of that stubborness in their fantasies, is more focused on being unpredicteable in how they interact with people and the social image they give off.
Of course, its not all black and white and some people have no subtype preference, some people have a slight prefernce and others a clear-cut one.