Side Decks are currently being tested by Bandai for Main Events.

I am happy to announce that digimoncard.io now fully supports Side Decks within our Deck Builder. This will allow you to build a deck with a 10 card side deck alongside it. 

Side Deck Details:

• Players will be permitted to bring their main deck of 50 cards and a side deck of up to
10 cards.
• Side decks may include main deck and Digi-Egg cards.
• The limit on copies of cards with the same card number that can be included applies
to the total cards in your deck and side deck. If rules or effects change the limit on
copies of cards that can be included, the limit applies to the total cards in both your
deck and side deck.
• The contents of the side deck are considered a secret area and should not be shown
to the opponent. Both players should count out their side decks in view of their
opponent to confirm how many cards are in there at the start of each match. Your side
deck may be returned to a deck box for safekeeping, which must be kept on the table
in view of both players.
• After the end of each game in a match, players may choose to swap cards into their
deck from their side deck. The same number of cards must be swapped between each
deck. The quantity of both decks must remain the same before and after siding.
• Please keep the time between games to under three minutes. All preparation for the
next game and exchanging of cards between main deck and side deck must be carried
out within these three minutes.
• Players may be penalized if a judge determines them to be time wasting between
games.
• Before the start of each game, if a player has a side deck, they should present it to their
opponent face down. The number of cards in the side deck is public information, but
a player is not required to reveal how many cards were swapped between decks.
• Players are not permitted to review their side deck contents during a game.
• At the end of the match, players must return their decks and side decks to their
original states.
• If a player receives a Game Loss penalty prior to the start of the first game, players are
not allowed to switch cards out from their side decks before the start of Game 2.

Ref: https://world.digimoncard.com/event/fest_2022/pdf/modified_rules.pdf