If a player is allowed to bring a card from outside the game into a Commander game, that player can’t bring a card into the game this way if it has the same name as a card that player had in their starting deck, if it has the same name as a card that the player owns in the current game, or if any color in its color identity isn’t in the color identity of the player’s commander.
April 17, 2020 · Ikoria
rule 903.11a not present in this release
July 13, 2018 · Core Set 2019
Brawl is an option for a different style of Commander game. Brawl games use the normal rules for the Commander variant with the following modifications.
July 22, 2016 · Eldritch Moon
rule 903.11a not present in this release
January 16, 2016 · Oath of the Gatewatch
A player that’s been dealt 21 or more combat damage by the same commander over the course of the game loses the game. (This is a state-based action. See rule 704.)