I wondered that too.  I was reading about the four temperaments in Please Understand Me.  In the book, it goes by the strongest motivation to fulfill needs of the temperaments.  So I think it's divided by needs.  The NT needs to be knowledgable and competent, the NF needs to "become" or embark on a life-long search for self, the SJ needs to be useful and the SP just needs to be free to experience.