I don't think you read Sierra's question right. I read his words to describe a coverplate, whether metallic or nonmetallic, that one commonly sees as the flush coverplate on drywall (or plaster) inside occupancies.
You seem to equate a "plate" and a "raised cover" as identical, but they are not.
I think they're asking if using a plaster ring and coverplate (wheter metal or plastic) on a surface mount metal 4 square box, is a code violation (like some goofballs who are too cheap to go and get an industrial raised cover for it an put it on like it should be ).
JAP>