It should be "models-of-peano-arithmetic" rather than "models-of-pa". For two reasons: one is that for people remote to the topic, "pa" does not necessarily ring a bell. Second, when writing a tag to a question, each word makes the system propose the tag where it's a subword. So "peano" would propose the unabbreviated one, not the abbreviated. In general, it's better to avoid abbreviations as much as possible in tags. — YCor yesterday