"In Typology 101, we provided an overview of the eight Myers-Briggs preferences (E, I, S, N, T, F, J, P). While the preferences can be helpful for determining your personality type and gaining a basic understanding of the ingredients that make up the types, they do not give us the whole story. Among other things, we also need to explore the eight functions. In this post, we will do just that, providing an introduction and overview of the eight functions."
I'm not sure in what area he is a doctor in, can't find that information, but he surely uses that title to gain pathos to sell his theories. Who can blame him for trying to make a living? Good resource though I think.