Your pool of ability cards is the set of cards you can choose to add to your hand when you start a scenario. The cards in your hand are the cards you actually play the scenario with and their number is limited based on the class you are playing. When you are starting out with a level 1 character, your pool consists of all level 1 cards, plus the three "X" cards in your deck as well. All other cards are unavailable to you. When you level up, you can take a single card of your new level or lower from the deck of unavailable cards and add it to your pool. In this manner, a level 9 character will add eight more cards to their pool over the course of leveling up from level 1.