RU

“RU (RarelyUsed) is Smogon’s third usage-based tier. All Pokémon that are not OU or UU by usage and are not banned to Ubers, UUBL, or RUBL can be used in RU, including Pokémon from NU and PU. While any of the aforementioned Pokémon can be used in RU, only those Pokémon that make up at least ~4.52% of total weighted usage in the tier are designated ‘RU.’ A Pokémon is truly ‘RU’ 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). In addition to its subforum and public room on Pokemon Showdown, RU also has a very active discord server.” -Smogon.com