400.11. An object is outside the game if it isn’t in any of the game’s zones. Outside the game is not a zone.
400.11a Cards in a player’s sideboard are outside the game. See rule 100.4.
400.11b Some effects bring cards into a game from outside the game. Those cards remain in the game until the game ends, their owner leaves the game, or a rule or effect removes them from the game, whichever comes first.
400.11c Cards outside the game can’t be affected by spells or abilities, except for characteristic-defining abilities printed on them (see rule 604.3) and spells and abilities that allow those cards to be brought into the game.