There's a formal tag used in this question that would possibly be more clear if it were renamed to a noun such as "formal language" or "formality". Should we change it, create a synonym, or leave it be?
+1 for formal-language
Comparing with formality and register:
- formality sounds very technical, and isn't as immediately comprehensible as formal-language. formality also may refer to other things.
- register includes all levels of formality, and it should discuss about each register itself, no the topics within the register, e.g. it should talk about how formal registers compare with others, or how they function as registers, not how things should be done in the formal registers
About whether to make a synonym or just re-tag the question, I would favor re-tag. This should always be the preferred choice if you are confident the tag wouldn't appear again. Luckily with the current small size of this site re-tagging is very easy, and more importantly SE's tag autocomplete would redirect new would-be formal tags to formal-language, and hopefully this can serve to unify all future questions of this topic under this one tag, without the need to use synonyms.