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Myst, j/p in socionics refers to the dominant function being rational or not. eg. SiTe is ISTp, because Si is irrational. The J/P in MBTI isn't defined that way, it considers both the dominant and auxiliary functions, which ever one is extroverted determines if it's J or P. eg. SiTe, the extroverted function is Te which is thinking, which is a judging function. So it becomes ISTJ.
For ISTP the P comes from Ti
Se, extraverted sensing, which is a perceiving function
ISTj in socionics, the J comes from
TiSe, introverted thinking, which is a rational function
That's how you get ISTP/ISTj as a single type