Ahh, you found me... Run!
(...) an encounter starts at the beginning of a campaign and ends after someone has won. Therafter going to encounter 2.
More or less this: Each quest is comprised of a number of individual encounters (normally 1 or 2), so you can use it each time you set up a new level.
So, a campaign consists of several quests (each with an unique name), and each quest consists of normally 1 or 2 encounters (different map layouts). At the start of each of these encounters, you can use the card.