many are unused as 'placeholders' while others are unused simply for being unfinished, which mostly are the ones for avatars or other player-centric things like that. The placeholders exist as padding for future-proofing replay codes up to a certain point to so the code interpretations don't need to check for version numbers and have a different reader for each, but probably could have made even more placeholders and still eventually used them all.
These placeholder types are '0' while the ones for avatar are '1'.