>>54932847Simply put, OU allows for the most amount of playable Pokemon that aren't obviously box legendaries or anything else on a whole other level to everything else by design (Mewtwo, Arceus etc.) but obviously not the most amount of viable Pokemon. The power level is set at around psuedo or 600BST legendary level and working from there. OU meta shifts occasionally mean that Pokemon with ~450BST end up becoming highly usable as niche but reliable counters to what's currently strong.
Naturally, tiers then form around usage and a Pokemon's overall power level, with Smogon's aim to make it so as many Pokemon have a place in the game, even if this involves making tiers where the accepted power level is significantly lower. What was once Ubers, OU and UU has extended to have NU, RU, even ZU and now UUbers to deal with the things that are too strong for OU but can't handle Ubers.
