The special field or specialty exam moves closer to the dissertation research. It may include a “narrative describing … educational background.” Done well, such a narrative can show how someone got to the burning issue that will drive the research.
If a student is already doing the kinds of professional writing (and the accompanying professional activities, research, scholarly inquiry, theorizing, etc.) that we are working to help them learn how to do, then it makes sense to certify that such is the case through examination of a portfolio of their work, rather than devising a separate exam. The portfolio is then the student’s evidence that she or he is doing this professional work.
The writing (or other materials) in the portfolio ought to show more than potential, that is, we should not have to infer that someday the student might be able to write a journal article, a book review, a proposal, or whatever, but rather that the works are already there. On the other hand, it’s unrealistic to expect that the works already be published or otherwise accepted into the professional literature.
Faculty usually expect a portfolio to include work done as a graduate student in the department, but perhaps also work done elsewhere. It could include work done for various purposes (research projects, independent studies, courses, etc) as well as work produced specifically for inclusion in the portfolio. There is an oral defense at the completion of the papers.