In many rules sets, there is the concept of rules that are essential to the game, and optional but official rules that can be used if desired. Contrast with house-rules, which are fan created additional rules.
Optional rules can completely replace certain parts of the rules set, or merely add to the existing rules to make them more complex, complete, and/or well-rounded. Many times these rules are presented in cafeteria format, i.e. pick and choose what part of the additional rules fit the campaign.
This tag should be used to discuss implementation, effects, and/or changes engendered by the use of optional rules in the basic rules set.