Royal Writ cards come in 2 varieties: Charters (half color seal) and Mandates (full color seal).
Mandates grant you resources (from the general supply) or attack strength, and may require you to either turn in a good (outlined by a square) or achieve guild status in order to realize the award.
Charters grant you end game veneration points based on completed objectives. Mandates are already sealed by the king and can be played and discarded at any time; however, in order for charters to become valid, you must obtain a king’s seal and place it on the card at the END of the game.
You may not have more than 3 Royal Writ cards at a time, but may discard unused cards to the bottom of the deck at any time.