Rulebook page 44
"When a character reaches a new level, add a new card to their active card pool. The card selected must be from the character's class and have a card level equal to or less than the character's new level."
Thank you. There are 2 other instances in the book which just state:
"As characters level up, higher level ability cards will also become available. Even though players have access to more cards, the maximum number of cards they can take into battle (hand limit) remains the same. Therefore, at the beginning of a scenario, players must select a number of cards from the character’s available card pool equal to the character’s hand limit."
I just couldn't find the right entry.