Esther- I just went and looked at that page. Given where he uses that symbol(the bracket with the letter R inside) I’m gonna take a wild guess. The R with the bracket means right hand and I’m guessing that he is emphasizing the fact that the right hand plays on the beat, meaning that the rolled chord starts before the beat so that the melody note will land exactly on the beat and not after it. I’ll be curious to see if anybody can find Grandjany’s own explanation.