R symbolizes Christ king. It is the first letter of latin word Rex. Jesus is king.
The medieval world assigns to Christ the two essential roles King and Judge. He is King for his omnipotent power over the kings and feudal lords. In the nimbus of Christ Pantocrator presiding over the magnificent tympanum of Conques can be seen the two Christ attributes. The nimbus has two overlapping planes with letters inscribed. At the lowest level with four sectors is read IV δ Є X. At the upper plane with three segments is read R E X.
Santa María de Moreruela (Zamora), Nuestra Señora de Veruela (Zaragoza), Santa María de Poblet (Tarragona), Santa María de Huerta (Soria), Iglesia de San Nicolás en el Castillo de Monzón (Huesca).
Read more in The stonemason’s signs in the Romanesque churches in Spain.