That iff for if-and-only-if is maybe be understood in conputer science and logic, and also bears an implied symmetry ("and not when not").
It is a useful operational term, with quite expressive power. But it needs always explanation, and the more or less implied exclusion of the negative case (when that is the case) needs also to be explained.
This means that best would be to use a foot note with a clear wording.
This function returns True iff1 the value of foo is larger than bar.
1) if-and-only-iff = necessary and sufficient condition. And not so when bar less or equal to foo.