Historical Rules · 7 February 2025
Rule 726.2a
Official source text from the 7 February 2025 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.)