A dichotomy is a binary trait that divides phenomena into one of two opposite categories. Dichotomies are a necessary attribute of any typology. For example, Jung's Typology divides people using four dichotomies: extraverted / introverted, rational / irrational (or "judging / perceiving" in the Myers-Briggs interpretation), thinking / feeling, and intuitive / sensing. There are 16 possible combinations of the axes or poles of these four dichotomies.
Dichotomies in Jung's Typology and in socionics exist side by side with functional models and are generally recognized to be a result of the position of various IM elements rather than an independent property. For example, rationality / irrationality and extraversion / introversion are defined by the leading function. Sensing / intuition and thinking / feeling ("logic / ethics" in socionics) are determined by which of these functions is present among the first two functions.
Therefore dichotomies are fundamentally built into the rules of function positioning. For example, you cannot have sensing and intuition as your first two functions, or both ethics and logic.
Compared to the Myers-Briggs Typology, socionics employs quite a few additional dichotomies. Some socionists, like Viktor Gulenko, integrate into their systems dichotomies from outside socionics, especially to characterize intratype differences.