Basically,
Te, Ni always directive;
Fi, Ne always informative;
Se, Si, Ti, Fe, can go either way, depending on the other preferred function.
When those last four are together (SeTi, SiFe), then it's the rule for S's: S+T=directive, S+F=informative.
Of course, E/I are determined by which of those functions is dominant.
As for whether this is based on the functions, most would probably assume so. But I once proposed an idea where the temperament and Interaction styles factors were the underlying basis instead:
http://www.typologycentral.com/foru...cs/36880-types-neurological-binary-codes.html