Battle Clauses are standard rules that trainers should follow in the competitive metagame. These first six are the most commonly used clauses. Here are the explanations of each clause and why they are used.
Evasion Clause - You and your opponent can not use evasion increasing moves like Double Team and Minimize. This clause does not include Accuracy reducing moves, as this is considered Pseudo Phazer. This clause is used because it simply unfair to have a +6 evasion by using evasion increasing moves since all attacks are lead to chance and not actual skill.
Legit/Legal Clause - None of the pokemon can have illegal movesets, altered or changed stats from what that pokemon wouldn't normally have,and over 510 EVs. An example of an illegal moveset would be would be a Magnezone with Flamethrower or Salamance with Ice Beam. However this clause is a little more flexible as to location as no one can really tell if your pokemon was hatched from an egg or caught at a different location. This doesn't really matter as long as the pokemon wasn't altered in any way from its normal set.
OHKO Clause - You and your opponent can not use moves that causes instant one-hit knockouts. Some examples would be Sheer Cold, Fissure, and Guillotine. This is used for basically the same reason as the Evasion Clause since these attacks are purely chance related and not skill.
Sleep Clause - Only one pokemon can be put to sleep (on your or your opponent's side of the field) at a time. The only time your opponent can use another sleep move is when your pokemon either wakes up or faints, then your opponent can use a sleep move again. The move Rest does not count toward sleep clause, so if your opponent were to put a Pokemon asleep because of Rest, you can still use a sleep inducing move on your opponent. If you put to sleep a Pokemon that has the ability Natural Cure and your opponent switches out this Pokemon, you can put to sleep the next pokemon without breaking this clause. For obvious reasons an enitre team put to sleep has almost little to no chance of making a come-back unless the entire team focuses around being asleep which is highly unlikely. So this clause is used to keep status ailments at a minimum.
Species Clause - You and your opponent can not use more than one of the same pokemon. This means that you or your opponent can not use two of the same pokemon in general. Trainers don't want to see 6 insanely overpowered pokemon on a team. Plus it diversifies the battle more.
Uber Clause - You and your opponent can not use ubers in a battle. This is generally used because ubers have insanely high base stats that run up to 150 or more. Ubers include Mew, Mewtwo, Lugia, Ho-oh, Deoxys(all forms), Kyogre, Groudon, Rayquaza, Latios(Latias is not an Uber), Palkia, Dialga, Manaphy, Giratina, Darkrai, Arceus, Wobbuffet, Wynaut
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Now for rules that usually aren't used in standard competitive battles but you might be asked to follow them. As you read through them, you'll understand why.
Focus Sash/Choice Item Clauses - You and your opponent can not use more than one Focus Sash and Choice Items (Choice Scarf, Choice Specs, Choice Band) in a battle. This is because Focus Sash insures that any pokemon holding this item with always have to be hit twice in order to faint. So if every pokemon on a team has one, then it is an unfair matchup since every pokemon has to be 2HKO. Choice items also raise stats by x1.5 which can make a big difference in a battle if every single pokemon is holding a Choice item. While the Choice Item Clause isn't used as much as the Focus Sash Clause, keep in mind the two usually go together.
Freeze Clause - You and your opponent cannot use moves that can freeze you after a Pokemon has already been frozen. You will have to wait for the frozen Pokemon to faint or thaw out before you can use another move that can possibly cause a freeze. Even though that it is a rare occurance to freeze your opponent's Pokemon, not many people feel like losing due to a freeze. A few moves that cause freeze would be Ice Punch, Ice Beam, Blizzard, and Powder Snow. This isn't normally followed as Ice Beam and Ice Punch are the most commonly used Ice attacks in battles.
Item Clause - You and your opponent can not use Pokemon that hold the same items on their team. Leftovers is usually excluded from Item Clause, but it is up to the person who uses the Clause. Items that have the same effect, but are different items entirely, do not count as the same Item. This is basically the same as the Focus Sash/Choice Item Clauses.
Luck Items Clause - You and your oppnent can not use Items that rely on luck to be activated. Some examples are Focus Band, Bright Powder, and Quick Claw. This is used occasionaly because luck shouldn't decide a battle.
Perfect IV Clause - You and your opponent can not use Pokemon that have perfect IVs in all stats. Since it is so rare to get Pokemon with 31 IVs in all stats, many would consider it hacking if you have a Pokemon with 31 IVs in all stats. This is not normally asked of or seen, but it is implied as it may as well be part of the Legit/Legal Clause, but at the same time no one can really tell if someone's pokemon has perfect IVs. This isn't normally seen anymore since a lot of trainers that use Pokesav or other hacking devices make teams that are legit and still have 31 IVs in all stats. Tthese people usually create teams for other players, so everyone nowadays usually has perfect IV pokemon.
Self-KO Clause - You and your opponent can not use moves that KO yourself when it is your last Pokemon vs your opponent's last Pokemon. This is to avoid draws in battle. Some examples are Self-Destruct and Explosion. Destiny Bond does fall under this clause if it works. However if a Self-Destruct move were to be used, the user of the attack would be counted as the loser.
Skarmbliss/SkarmCressBliss Clause(Strict Clause) - You and your opponent cannot use Skarmory and Blissey (and Cresselia, if you are using SkarmCressBliss Clause) on the same team. You can only have one of them on your team, whereas the Strict Clause doesn't allow the use of any those pokemon on your team. This is because properly trained Skarmory, Blissey, and Cresselia are insanely good walls and are near impossible to pass. However some teams have an easier time getting passed these walls than others, so the Strict Clause is usually in effect.
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