Historical Rules · 2 February 2024
Rule 725.2a
Official source text from the 2 February 2024 Rules release
As a subgame begins, if one or more supplementary decks of nontraditional cards are being used, each player moves each of their supplementary decks from the main-game command zone to the subgame command zone and shuffles it. (Face-up nontraditional cards remain in the main-game command zone.)