GraphicRaycaster Blocking, priority - uGUI

How does the graphic raycaster blocking of various objects and priority work?

Note to SaraCecilia, who both locked and closed the previous question, I'm not sure how to phrase this more specifically (?) - would this be more specific? How does priority relate to blocking? - it's basically a rewording, and I'm not even sure if they are related.

The documentation does not explain how priority affects different blocking settings.