There are joint appointments (split FTEs) and courtesy appointments (0 FTE lines).
Joint appointments are when two departments equally (or not) contribute some of their dedicated faculty lines to a position. For example, Physics gives 0.5 FTE (half of a faculty slot) and Chemistry gives 0.5 FTE and they bring in a person who gains the title of "Assistant Professor of Chemistry and Physics." Normally this person would have duties in both departments.
There are also courtesy appointments, also called 0 FTE appointments. If the above person was brought in 100% by Chemistry (1.0 FTE) but for various reasons wanted to occasionally teach a cross-listed course in physics, serve as a chair on a couple of doctoral committees, or other sundry reasons, the Physics department could decide to add this person to their affiliate faculty list. It would not cost them any lines (which is why it is 0 FTE) and would solve some administrative requirements.
Note that some university programs are entirely consisted of 0 FTE positions and have no faculty lines of their own. Women's Studies is a good example of a program that on many campuses is entirely 0 FTE driven.
In the case of the example given by the OP, the person is "Professor of Bioengineering, Genetics, and Medicine (and Computer Science, by courtesy)." This would seem to indicate that he has three 0.33 FTE lines (or 0.5 FTE, 0.25 FTE, 0.25 FTE; etc. etc.) and one 0 FTE courtesy appointment.