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Silver Bottle Cap
From Pixelmon Wiki
The Silver Bottle Cap is an item that maxes out one of the six stats (HP, Attack, Defense, Special Attack, Special Defense and Speed) of a level 100 Pokémon. When you right click a level 100 Pokémon with it, you will be able to select what stat do you want to max.
The Silver Bottle Cap is obtainable via fishing and as a tier 3 Special drop.
Bottle caps only effect the ivs for battling purposes it doesn't affect the type of Hidden power and it doesn't affect breeding.
