>>48150973I think they mean that Gen III introduced two new regions, so Kanto three years later wouldn't count since it's an old region. The post is obvious bait though and I agree that Orre shouldn't count. Strictly speaking, Orre is a spin off region like Fiore, Almia and Oblivia from the Ranger series or Ransei from Pokemon Conquest. If I'm not mistaken, Colosseum was originally intended to be the Gen III equivalent of Stadium 1 & 2 just like Battle Revolution in Gen IV. The RPG-esque story mode and ability to trade Pokemon back and forth with the main series games (and, in the case of XD, capture wild Pokemon) is probably why some people consider them Main Series. Bulbapedia considers the Stadium series as side series rather than spin-offs like Snap, Mystery Dungeon, Ranger, etc., but they're still not canon as they were made by Genius Sonority rather than GameFreak. They also don't have routes and lack a gym leader/island trial campaign and a Pokemon League.
