Excerpt for functions tag says at the moment:
Elementary questions involving functions.
From this it is not entirely clear what kind of questions the tag is intended for. But I definitely think that some question tagged by this tag do not correspond to this description. Maybe expanding tag-excerpt/tag-wiki could help but I did not want to this without discussing the issue here first.
I'll post an answer about what I think was the intended use; please, if you think that I've forgotten some important things or if you have better suggestions, please post them as answers.
I propose that if we reach some consensus about when this tag should be used, we can edit tag-excerpt to reflect this. (In order to help users to use this tag correctly.)