Tiers are an unofficial method of classifying Pokémon within a particular competitive battling format based on their relative usage.
Individual sites, users, or organizations may publish tiers, but the most prominent tier lists are produced and published by Smogon, usually based on usage stats from unofficial battle simulators, currently Pokémon Showdown. The battle simulator Pokémon Online also previously published its own tier lists.
Tiers are defined separately for each generation. Tiers for the latest generation of games are typically updated more often than older generations, due to those usually being the most played formats.
Each tier functions as its own format, where only Pokémon that are not commonly used in higher tiers can be used. By categorizing Pokémon in this way, Pokémon that would normally be outclassed in higher tiers' formats can still be useful in lower-tier formats.
In addition to usage-based metrics, some Pokémon are banned from a tier due to being considered unhealthy for that tier's metagame.
If a Pokémon is meets a certain threshold of usage within a particular tier, it is classified as being within that tier, and cannot be used in any lower tiers. A Pokémon's classified tier is only its lower limit; it can still be used in any higher tier. Some Pokémon may still be useful in higher tiers despite their classification, but are simply not widely used.
If a Pokémon has multiple forms that it cannot switch between within battle, those forms may have usage calculated separately from each other. However, if the form difference is purely or mostly aesthetic, those forms are often grouped together and tiered on their own.
If a Pokémon is banned from a tier, it is placed in a special BL (borderline) group for that tier, meaning it cannot be used within that tier or any lower tier, regardless of its usage.
Because the metagame shifts over time, as well as Pokémon being banned and unbanned, Pokémon's usage within a given tier will vary over time. At certain points in time, the publisher of the tier system will update which Pokémon are in which tiers based on current usage.
Different generations have different numbers of tiers. Newer generations have more Pokémon and more interest, so typically have more tiers as a result. Publishers of tier lists usually only create new tiers when there is enough interest in them to justify maintaining that new tier.
Tiers are normally ranked as follows.
- OU (Overused)
- UU (Underused)
- RU (Rarely Used)
- NU (Never Used)
- PU (expression of disgust)
- ZU (Zero Used)
Some publishers of tier lists used the LU (Little Used) tier in place of RU. However, due to the prominence of Smogon's tier list, Smogon's terminology has dominated most tier lists.
Each tier functions as its own format. Each of these tiers has its own banlist, normally referred to as the tier's name appended with BL (e.g. UU-BL). However, OU's banlist has its own unique name, Ubers.
Unlike the other banlists, Ubers functions as a format itself. Due to functioning as format itself, Ubers itself has its own banlist, for Pokémon that considered too unhealthy even within its own metagame. Unlike tiers, Ubers is not based on usage, so usage within Ubers does not result in a Pokémon ceasing to be in the OU tier.
In addition to banning specific Pokémon, each format may also ban certain moves, Abilities, items, or even the usage of particular game mechanics (such as Dynamax). They may also issue bans on certain combinations of these, such as a party having two particular moves at the same time, although this is considerably more rare.
Playing without any restrictions (or only very specific rules such as the "Endless Battle Clause") is typically referred to as "Anything Goes".
Battle simulators typically offer tier-based formats by default, automatically enforcing the restrictions associated with them. To play with these formats in the games themselves, both players need to agree to the rules in advance, as in-game rule-creation options typically do not offer enough granular control over the rules to support these formats. Due to both the difficulty of enforcing these rules in-game and the availability of usage data, tier lists are usually based on usage within battle simulators.
Fans usually discuss tiers in the context of standard Single Battles. However, tier systems have also been published for other variants, such as Double Battles and Little Cup Single Battles. Tier systems for these format types function in the same way, but typically do not have as many tiers as standard Single Battles due to lower popularity of these format types.
Similarly, some sites (such as Smogon) create their own custom Pokémon, and have format types that include those Pokémon with their own tier lists.