I don't recall the term cropping up. It's certainly not used as an actual monster classification in the rules.
I've noticed it as well, anyone has an idea why it's the case? There are many instances where it's specified "fiends, celestials, fey, elementals" (like Abjure the Extraplanar). Would be much easier with just "Outsiders" and the creatures having appropriate tags (Abishai: Outsider - fiend (devil) or something like that).
