Competitive Pokémon Battling, also called the Pokémon 'Metagame', is highly unbalanced at its core, and as a result the bulk of the battling community live by guidelines that help to bring balance. One of those guidelines is the division of the Metagame into tiers, and Pokémon are assigned to these tiers based on: Use and Power.

Pokémon are assigned to several tiers: Ubers, Standard aka Overused (OU), Borderline (BL), Underused (UU), Never used (NU).

Standard/Overused (OU)

Standard, as it implies, is the general and 'normal' environment for Competitive Pokémon Battling. Ubers are banned from this environment but everything else is okay. This is where most people battle and the likes of Salamence, Garchomp, Tyranitar and Blissey can freely be used. The term 'Overused' is also used for this environment, since this is where the most frequently used Pokémon will battle.

Standard, Borderline, Underused and Neverused Pokémon are all freely accepted in this environment.

Underused (UU)

As the name implies, this is where Pokémon who aren't used often are battled. Generally speaking, Pokémon in this environment are 'weaker' than Standard Pokémon, and so this environment is used to battle them. By limiting this environment it allows for a balanced Metagame whilst granting a different experience to standard play. Pokémon like Altaria and Flareon make their home here.

Underused and Neverused Pokémon are freely accepted in this environment.

Never used (NU)

Pokémon in this environment are strictly speaking, the bottom of the barrel. These include Pokémon who aren't even used in the UU environment and the Metagame is considered somewhat of a novelty. Pokémon like Beautifly and Luvdisc exist here. Only Neverused Pokémon are accepted in this environment.

Borderline (BL)

Borderline is the home for Pokémon who are deemed too powerful for the UU environment. Generally speaking, Borderline doesn't have a true battling Metagame, rather its where 'Underused standards' are placed. Examples include Pokémon like Articuno and Entei, their power is clearly too strong for UU but they are rarely seen on Standard teams.

Were a proper Metagame for BL to exist: Borderline, Underused and Neverused Pokémon would be accepted in this environment.

Uber

Pokémon placed in the Uber environment are Pokémon deemed too powerful to be played in Standard. Generally their power is unbalancing, and if they were allowed in standard play they would over-centralise teams and limit the Metagame.

The following Pokémon are considered 'Uber' and are banned from standard play:

Mewtwo
Mew
Ho-oh
Lugia
Latias
Latios
Kyogre
Groudon
Rayquaza
Deoxys (all forms)
Dialga
Palkia
Giratina
Darkrai
Arceus
Manaphy
Wobbuffet

Some have called into question why Pokémon like: Jirachi, Celebi and Shaymin aren't on this list, especially when: the Lati twins, Manaphy, Deoxys and Mew are. The simple reason is because those Pokémon have power comparable to most Standard Pokémon, whilst the Lati twins, Manaphy, Deoxys and Mew don't.

Jirachi and Shaymin are limited by their shallow and predictable move pools whilst Celebi is limited by a horrible typing. When compared to Standard Pokémon they are balanced and as a result are allowed in the Standard battling environment.

The Lati Twins on the other hand have fantastic typing, a good trait and a great move pool, whilst Mew and Deoxys have unquestioned versatility and fantastic offensive (or defensive) prowess. Manaphy, despite having a fairly shallow move-pool, is near unstoppable with a Tail Glow under its belt. When compared to Standard Pokémon they are unbalanced and as a result are not allowed in the Standard battling environment.

A lot of people also question why Wobbuffet is on this list. To put it simply, its Trait combined with its stats make it too devastating to be used in the Standard environment. This may or may not be changed at a future stage, but as it currently stands, Wobbuffet is likely to be locked into Ubers.

As you can guess, all Pokémon are freely accepted in the Uber environment.

So, what does this all mean?

Generally speaking, battlers expect other battlers to abide by these guidelines. If one battler requests a “Standard Battle” then they would assume their opponents wouldn't be using any Uber Pokémon. If someone requests an NU Battle they would expect their opponents to only use NU Pokémon, and so on.

Guide written by Whillowwhisp