Questionable is probably not the right word. The idea is to find characters that you might be surprised are in your document. It's specifically helpful for zero

Questionable is probably not the right word. The idea is to find characters that you might be surprised are in your document. It's specifically helpful for zero width or invisible characters, which is replaces with their name. Future revisions might say "Most of your characters are European but you have this one Kanji character, let's highlight that" But for now it just highlights all non-ASCII.