UU

“UU (UnderUsed) is Smogon’s second usage-based tier. All Pokémon that are not OU by usage and are not banned to Ubers or UUBL can be used in UU, including Pokémon from RU, NU, and PU. While any of the aforementioned Pokémon can be used in UU, only those Pokémon that make up at least ~4.52% of total weighted usage in the tier are designated ‘UU.’ A Pokémon is truly ‘UU’ if a typical competitive player is more than 50% likely to encounter that Pokémon at least once in a given day of playing (15 battles).” -Smogon.com