There seems to be two views on what the letters "RNIO" mean in the Carlos Crivelli painting. I only have one reference for the answer: Dr. Liliana Leopardi, an expert in Italian Renaissance art, Assistant Professor of Art and Art History at Chapman University, who was gracious enough to share her knowledge with me through personal correspondence.
Scholars have been trying to figure out the inscription for quite a long time now. It is thought to be to an inscription that somehow refers to the commissioner's identity. As it happens to be painted on one of a shield.
Scholars are divided on the way that it refers to the commissioner's identity, Dr. Leopardi continues:
It could either be a reference to the name of the commissioner or the initials of a series of words that were meant to be the commissioner's motto.
The answer unfortunately is that scholars simply don't know the answer.