During the game you will recieve certain points in the game that will allow you upgrade regular cards to Premium Cards. These so-called Premium Cards are special upgraded cards that have special abilities to them. When you upgrade to Premium a prompt will appear after battle, this will allow you to pick a random card from your deck to upgrade to Premium. Premium Cards require less CP than normal cards, however after one use they are discarded from the deck, rather than regular cards being re-stocked.