This rank is reserved for someone who is heiress to the kingdom throne and therefore known as Crown Princess, or a ruler of a principality. The king and queen themselves are the prince and princess of any Crown Principality that may be in existence at a given moment.
The princess is accorded very high rank as a member of a kingdom's nobility. The Crown Princess ranks immediately after the king and queen, together with the Crown Prince, and is addressed as Her/Your Royal Highness. The princess of a principality ranks immediately after the Crown Prince and princess, together with her prince, and is addressed as Her/Your Serene Highness.
There are no principalities in the Middle Kingdom at this time, so the only princess at any given time would be the Crown Princess, also known as the Tanista.