Smogon has established [Six] clearly defined tiers. Metagames restricted to a particular tier automatically include Pokemon from lower tiers. In practice, this means that in OU play, Pokemon from the BL, UU, RU, NU, and [PU] tiers are allowed to participate, but Ubers are not.
Yes, you can use Pokémon from lower tiers (and all "un-tiered" Pokémon that aren't CAP, Limbo, Pokéstar Studio Pokémon/Missing no., or Mega Rayquaza) in higher ones, although, Pokémon in lower tiers are generally lower because Pokémon in higher tiers perform better and or to a better consistently than the lower tiers.
Source - A little outdated being from BW, but same concept. :P
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