I have noticed that there exists derivation tag.
It contains two questions at the moment, so it might be quite recent. Maybe it was created here.
The tag-wiki is empty, but it seems that the tag is intended for questions about derivation of some fact, steps in some derivation, etc. (Judging from the questions that have been tagged with this tag so far.)
Will such tag be useful? Do we need it?
It smells of a meta-tag a little.