Is that the right term, "sleeve"? I always thought of the "sleeve" on my 5.25" floppies as being the tyvek/paper thingie you stuck the floppy disk inside for safekeeping (ultimately putting both in a case/cabinet - which I still have several).

Maybe the thing I thought of as a "sleeve" (analogous to a album "sleeve") is actually called something else (maybe an "envelope"?) - I never really gave it thought until now, honestly.