In Western type theory, it was first outlined by Beebe, then, people discovered the Socionics stack. In Beebe's theory, the eight positions are essentially set by different complexes that associate with each function. And it's all "mirroring", based on, basically, the division of reality by ego consciousness.
So we will have a preference for one mode of taking in information (S or N), one mode of making determinations of what's right (T or F), and one dominant orientation for taking in or determining (the environment [e] or our own self-reference ). One of these functions will be "dominant" (the ego's main outlook on reality), and will take the dominant orientation. The other function will then fall to a lesser, "supporting" role of adding balance, and thus take on the opposite orientation.
Type is these two functions, and they associate with the ego's main complex, and a "supporting" complex. The other six are "reflections" and "shadows" of these two; meaning collections of the "split off" opposite function and/or orientation. The inferior is what's split off of the dominant, and thus the opposite function and orientation. Likewise, the tertiary is what's split off of the auxiliary. This yields the first four (which were the only ones dealt with by most MBTI theorists), but then the reverse orientations of these are just something else further split off. That's why they're the "shadows" (And in Beebe's model, associated with shadowy archetypal complexes).