Re: Swiss Psychologist Carl G. Jung: Can someone please tell me if/where I can find (the source) of where/when Carl Jung said/wrote about how in his clinical study of people, wherein he admitted that he could only make judgments (labels) on something like the "extreme versions" of people? Where, according to his "scientific humility" (for lack of a better phrase), he felt he could not rightly assess various persons to be extraverted, intuitive, thinking, etc. for example, unless they were more extreme in their discernible behavior? That is to say, some people under his study he refrained to make certain analyses, because he just didn't feel like he had enough evidence to support, etc.

I believe you are referring to Jung's discussion of Sigmund Freud's instinct-based theory and Alfred Adler's ego-based theory. This discussion is from Jung's book Psychological Types. However, Jung also discusses the limitations of applying totalizing theories in particular cases of specific patients in his book Two Essays on Analytical Psychology. His overall point is that there is always going to be a disjunction between abstract theory and reality, particularly when adherents to theory will not see beyond their own principles, as he believed was the case with both Freud and Adler:

The scientific tendency in both is to reduce everything to their own principle, from which their deductions in turn proceed. In the case of fantasies this operation is particularly easy to accomplish because . . . they . . . express purely instinctive as well as pure ego-tendencies. Anyone who adopts the standpoint of instinct will have no difficulty in discovering in them the "wish-fulfillment," the "infantile wish," the "repressed sexuality." And the man who adopts the standpoint of the ego can just as easily discover those elementary aims concerned with the security and differentiation of the ego, since fantasies are mediating products between the ego and the instincts. Accordingly they contain elements of both sides. Interpretation from either side is always somewhat forced and arbitrary, because one side is always suppressed.

Once a theory has been adopted as the overarching standard of psychoanalytic judgment, it is very easy to see patients in terms of extremes. Jung urges against this, using the example of fantasies, which pertain to both the ego and the instincts and thus will be distorted (because seen as overly "pure" or extreme) if seen in light of one theory only.
Jung is suggesting something like the idiom "if you only have a hammer, everything looks like a nail." If you only have a theory centered on extremes (such as extreme ego-determination), you will find the problem that fits your pre-made theory, not the truth about the analysand. Jung is arguing for greater subtlety and for a certain artful craftsmanship. In his Red Book, he pointedly argues for scientific humility as well.