1) Game design 2) Game twist 3) Game-changing invention 4) Locale-specific regulation 
House rules are rules applying only in a certain location or organization. Bars and pubs in which games take place frequently have house rules posted. For example, it is a house rule in United States Air Force officers` clubs that if an officer enters the club wearing headgear and is officially noticed (i.e., the bell near the bar is rung), the e....
A new or altered version of current rule used by a given group. This rule is not part of the published material for the game.
A change or alteration on official RAW agreed on by both/all players.
a rule that is used in a game only in a specific place, as a particular casino, or only among a certain group of players.