It is not to be used during a duel, but you can swap cards between it and the main and/or extra deck between the duels in a match
I'll highlight the game plans, key cards, and defining strengths of the top fifteen decks in the current metagame
Recommended cards: The Phantom Knights of Break Sword (extra deck), Raider's Knight (extra deck), The Phantom Knights of Silent Boots, The Phantom Knights of Torn Scales, The Phantom Knights of Shade Brigandine Madolche A sweet deck that leaves your opponent with a bitter taste, Madolche's are one of the archetypes to benefit most from the XYZ festival limitations
In many cases, it's said that the Base set of Missing-Rarity-Symbol is first printed
They play a big part of the story in FF14, rather than just being superbosses
Additionally, Pokmon-ex are immune to certain effects: for example, their Pok-Bodies are not deactivated by cards such as Space Center from EX Deoxys, but they are also excluded from the benefits of certain cards, such as Sitrus Berry from EX Unseen Forces, which cannot be attached to them