Since quite some time. Also a lot more since auto-selection was introduced. Whenever you have vocal, it will auto-classify it as such. There's a guide difference on how Vocal is used for album classification vs. product classification. Makes things more complicated and a bit out of sync.
Spoiler:
Album guide/tooltip:
Quote:
Vocal
Music that includes vocals
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Product linking:
Quote:
Vocal - A special categorization for vocal arrange albums. This overlaps arrange, so you don't have to include both classifications.
OP/ED/Insert - This is a special classification for albums that mostly contain Opening or Ending themes, as well as other insert or image songs.
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also slightly strange: "as well as other insert
or image songs", why group image which is most likely an unused song with op/ed & insert songs which are... used. This one looks more specified:
Quote:
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Prototype - Catch-all for unused tracks and image albums.
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And what are we going to do with vocal character songs CDs? Are they really "vocal arranges" for instance? Arrangements of what? They're just there so the voice actor can sing. Is the character song in a way an image song, as in 'unused'? It's different from op/ed (or insert song which does get used at some point in anime). Character song doesn't have to be "used". So are they classified as prototype/unused (at album classification) and prototype on product pages? Nope, they're classified as vocal.
So the practical use has apparently overriden product linking guide partially.
If there's a need for adjustment perhaps, it could very well be the linking products guide... or something.