Intuition and sensation, are the irrational/perceptive functions.

Thinking and feeling, are the rational/judging functions.

In an ISTJ, the rational/judging function of thinking, outweights the irrational/perceptive function of sensation, and therefore, the ‘J’, indicates that the judging function of thinking is dominant in this type. However, ISTJ is listed alongside IS(T), in the diagram, with the function represented by the bracketed letter described in the article as the ‘auxiliary function’, which is not the case, because, I reiterate, in the ISTJ, the fact that the rational/judging function of thinking supercedes the irrational/perceptive function of sensation in this type, is indicated by the ‘J’, pertaining to the dominance of the judgemental faculty, which is in this case, the thinking function.

Likewise, in the ISTP, the irrational/perceptive sensation function outweights the rational/judging thinking function, and hence the ‘P’, indicates the dominance of the perceptive sensation function.

Therefore, according to the description of the article: that the letter in brackets indicates the auxiliary, and not the dominant function, in that type, then the diagram ought to read as follows:

ISTJ = IT(S)
ISTP = IS(T)

Why the brackets/parentheses are considered necessary anyway I cannot fathom, as the ordering of the letters should suffice to demonstrate whether a thinking-sensate (J), or a sensate-thinker (P), is being indicated.

The diagram is therefore misleading as it explains that the function indicated in brackets in the Jung columns, are the ‘auxiliary function’, which isn’t the case, it’s the other way around.