Based on his theory, Meyers-Briggs developed the four letter codes to coincide with the functions, which subsequently became known as dichotomies. The final code simply shows whether you prefer to interact with the world with a judging or perceiving function. Nothing else. As stated before I can simply call myself Ti-Se which is Jung's theory, instead of ISTP.
I do have my thoughts on why it was necessary for Meyers-Briggs to create the codes, which has more to do with her mother's theory on how introverts interact with the world and coincides with the fact that she does not believe introverts show their real selves to the world. This allowed Socionics to muddle things because there is only an indication that Katherine Briggs met Jung briefly, but no indication that they discussed or he agreed with her theory for introverts. Thus he could easily have said that all perceiving types, whether they extravert or intorvert their dominant function would be considered Ps and all judgers, Js. In that case the Socionics theory would be correct. Nevertheless, I would still be Ti-Se in either system. As it stands the four letter dichotomies have taken on a life of their own, that when focused on moves readers further from Jung's theory.