if i'm not mistaken, formal post-bac programs refer to the programs most people think of when you mention doing post-bac work. informal programs refer to individuals taking undergrad level classes as a non-matriculant in order to boost their undergraduate gpa, without applying to (or being admitted to) any structured post-bac "program."
correct me if i'm wrong.
which, to answer your question, should make most of the programs on the aamc site formal post-bacs.