Custom Parts are used to improve the factors that judge a weapon's performance, such as damage and reload speed. Their names normally indicate what factors they alter, such as "Damage". There are also multiple versions of Custom Parts modifying the same factors; they are identified by numbers (e.g. "Damage Lv. 2").
In Revelations the Custom Parts are also divided into categories, including "Illegal"; "Rare" and "Legendary" alongside the normal Custom Parts.