I wrote the tag wiki for the set-phrases tag, which is used 11 times and reads:
Phrases with fixed words used as a single unit, many of which are idioms.
We also have a expression tag which I personally found a bit vague so I didn't tackle its tag wiki. Now that it has been written it reads:
A set phrase used commonly in the Japanese language
So does this mean the latter tag is in fact vague, or is it functionally a synonym for the former? Or do we just need to reword one or both of them to clear up how they really differ?