PU

“PU is Smogon’s fifth usage-based tier. All Pokémon that are not OU, UU, RU, or NU by usage and are not banned to Ubers, UUBL, RUBL, NUBL, or PUBL can be used in PU. While any of the aforementioned Pokémon can be used in PU, only those Pokémon that make up at least ~4.52% of total weighted usage in the tier are designated PU. A Pokémon is truly PU 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