Rule 100.4 · General
Each player may also have a sideboard, which is a group of additional cards the player may use to modify their deck between games of a match. Sideboard rules and restrictions for some formats are modified by the Magic: The Gathering Tournament Rules (found at WPN.Wizards.com/en/rules-documents).
Subrules
In constructed play, a sideboard may contain no more than fifteen cards. The four-card limit (see rule 100.2a) applies to the combined deck and sideboard.
In limited play involving individual players, all cards in a player’s card pool not included in their deck are in that player’s sideboard.
In limited play involving the Two-Headed Giant multiplayer variant, all cards in a team’s card pool but not in either player’s deck are in that team’s sideboard.
In limited play involving other multiplayer team variants, each card in a team’s card pool but not in any player’s deck is assigned to the sideboard of one of those players. Each player has their own sideboard; cards may not be transferred between players.