If you have backed a princess, you can play Succession Cards in your Domain. Succession Cards that you play this way do not get discarded, and remain in your Domain, counting towards your point total. You can play as many Succession Cards as you have in your hand.
Example: You have a Duke and an Apprentice Maid in your hand. Since you want to play a Duke card, on this turn you skip buying any cards, and decide to play the Succession Card instead. You play the Duke card, and thus get 6 Succession Points. Since the Apprentice Maid is -2 Succession Points, you do not play it here.