It might be naive, but isn't the aim of a code review to find out if there is a way to refactor the code in order to have it improved?
If this tag doesn't apply to all question, when should I use the refactoring tag?
Hmm, good point. I could see the tag used for questions that want a refactoring reviewed. For example, a post could show the code, show a refactoring of the code, and ask if the refactored code was equivalent to the original.
However it is redundant many of the times it is used here. There are two questions that have refactoring as the sole tag, so I can't destroy it outright. Feel free to suggest edits for posts you feel the tag is useless, and I'll start going through myself in small batches so that the front page doesn't get disturbed too much.
They don't really add anything to the question and mainly served as "separate requests" for a review (site policy also states that anything is fair game). This would especially be problematic if someone were to try to "pick out" which meta tags fit best, and they simply wouldn't work if the question needed five language/technology tags.