What substantive title is held by the heir apparent to the throne of Spain?

The title originated in 1388, when King John I of Castile granted the dignity – which included jurisdiction over the territory to his first-born son Henry. In an attempt to end the dynastic struggle between the heirs of Kings Peter I and Henry II of Castile, the principality was chosen as the highest jurisdictional lordship the King could grant that had not yet been granted to anyone.