What is significant about scouts reference too boo as mr Arthur?

Scout's reference to Boo as "Mr. Arthur" signals her growth and understanding of Boo Radley as a person deserving of respect and dignity. It signifies a shift in her perception of Boo from a mysterious, shadowy figure to a neighbor worthy of recognition and regard. It also reflects Scout's empathy and ability to see beyond superficial appearances to recognize Boo's humanity.