Which rule prevents having extra cards like a promo in your deckbox at a tournament?

Ruling: There is currently no rule in Riftbound that prevents carrying extra cards in your deckbox at a tournament.

Nuances:

• Tournament rules in China (for city challenges) may be slightly different and may have stricter policies
• Carrying extra cards in the same sleeves as your deck is asking for trouble even if not explicitly prohibited
• Rules are subject to judge appreciation and may change later if problems occur at major events
• In other games at High OPL, any card in your deckbox is assumed to be part of your deck, but this is not currently the case for Riftbound