As the rulebook states, you can only use cards in your deck if you have at least one Bakugan of that Faction on your team of three. For example, if your team is Pyrus Dragonoid, Darkus Dragonoid, and Ventus Dragonoid, you can only use Pyrus, Darkus, and Ventus cards in your deck. But what if you send your Darkus Dragonoid into a Brawl and only have Pyrus cards in your hand to power it up? Don't worry; you can use any cards in your deck on any of your Bakugan. Just make sure to follow the deckbuilding rules.