Q: What is that new square button for when I edit a character on stage?
A. That button is new in 5.73! It's the "Layer Changer". Now, you can move characters to be in front of or behind other characters! This will help make sure that background characters really are in the background, instead of being super small in the foreground.
Right now, it doesn't have a number in the icon, but very soon it will! You'll be able to tell exactly on what layer each character is, as indicated by that number.