What is an affiliate professor? The answer to this question seems to indicate that an affiliate professor is effectively just a member of the department for administrative purposes. If that is the case, why is George H. Miley both an affiliate professor and a professor at the University of Illinois at Urbana-Champaign? Presumably his status as a professor would already grant everything that is granted to an affiliate professor.
(The above links to an old webpage, but the current one seems to say something similar, he is both an affiliate &amp; an emeritus professor.)
In the example you give (https://grainger.illinois.edu/about/directory/faculty/ghmiley), the person is &quot;professor emeritus&quot; in one department, and &quot;department affiliate emeritus&quot; in another.
I can't speak to that university in particular, but in certain universities, a professor in a given department can only take on graduate students from that department.  Affiliate status with another department allows a professor to take on students graduate students from that department as well.