The letters E or I, S or N, T or E and J or P are used to designate which of the opposite sides of a respondent's nature are preferred.
The intent is to reflect a habitual choice between rival alternatives, analogous to right-handedness or left-handedness. One expects to use both the right and left hands, even though one reaches first with the preferred hand.
Similarly, everyone is assumed to use both sides of each of the four dichotomies but to respond first, most often, and most comfortably with the preferred functions and attitudes.
Isabel Briggs Myers, Mary H. McCaulley, Naomi L. Quenk, Allen L. Hammer