Formation of the 16 personality types

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None of Jung’s eight types occurs in its pure form. The eight types combine with one another in specific ways, resulting in 16 subtypes (rather than 64, as one might assume by multiplying 8 by 8).

A psychological type is a combination of various psychological functions. In his work "Psychological Types", Carl Jung divided all eight function into two groups: RATIONAL (or judging) and IRRATIONAL (or perception). 

Sensation (S) and Intuition (N) are irrational functions (or perception)
Feeling (F) and Thinking (T) are rational functions (or judging)

When we say ‘Se type,’ we mean a type whose main function is the Se function, although other functions are also present in this type. One of the functions always surpasses the others in strength, speed of reaction and meaningfulness of this reaction. It is called the main one. Carl Jung discovered that, within the structure of the human psyche, the function ranking second in strength after the main function is the auxiliary function:
“Accurate investigation of the individual case consistently reveals the fact that, in conjunction with the most differentiated function, another function of secondary importance, and therefore of inferior differentiation in consciousness, is constantly present… Experience shows that the secondary function is always one whose nature is different from, though not antagonistic to, the leading function : thus, for example, thinking, as primary function, can readily pair with intuition as auxiliary, or indeed equally well with sensation, but never with feeling… As a judging function feeling successfully competes with thinking.” (1)

1 - Carl Jung “Psychological Types”, 1921. Chapter X. 11. The Principal and Auxiliary Functions