In Socionics, it is not required to distinguish these categories (only the types, Schizotim/Cyclotim), even when the two mentioned attitudes the most notable for some, their naming is inherited from Jung, but that's all about this distinction. This is a dichotomy like any other. The rest is the descriptions of the IM elements, their systematization is accomplished differently in Socionics. I agree though that it is one of the most difficult things to understand how Rationality (of a type) emerges from the association between Extroversion and Dynamicality in the Base function but nevetheless, as long as you don't deny the existence of any of the three IE distinctions, there's absolutely no problem to see how they are structured in the Socionics theory of IM.
Remember that Aushra herself denied that the IM elements (the topic of the thread, not types) can not be separated into Rational/Irrational:
http://www.the16types.info/vbulletin...841#post725841. And this makes sense, like in the case of Extroversion, Rationality is
an attitude (of a person), it is unapplicable to information itself. How can one say "this information is Rational"? It makes no sense.
I insist though that one checks this, because the functions flipped over one distinction are not similar merely on paper. There are a lot of instances where one can see the common properties between j and p functions, perhaps the most obvious is the similarity between Se and Te (Bodies, External): pragmatism, practicality, concreteness, empiricism. Where they differ is Static/Dynamic, see the distinction between inherent and emergent qualities (e.g. the untested steed VS the successful jade).