This rank is reserved for someone who is heir to the kingdom throne and therefore known as Crown Prince, 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 prince is accorded very high rank as a member of a kingdom's nobility. The Crown Prince ranks immediately after the king and queen, together with the Crown Princess, and is addressed as His/Your Royal Highness. The prince of a principality ranks immediately after the Crown Prince and Princess, together with his princess, and is addressed as His/Your Serene Highness.
There are no principalities in the Middle Kingdom at this time, so the only prince at any given time would be the Crown Prince, also known as the Tanist.