In answer to your query, if you're adding an overlay on the right-hand page and it overlaps onto the left-hand the page, if it's over 95% on the right hand page, you'll be warned about it, unless you've checked that box.
When you're warned, you'll get a message asking if you want the overlays to be cropped, and put entirely on the right, or kept as they are. If you select "Ignore", they will be kept as they are and the box gets checked.
This warning has been added because an overlay that extends onto the left-hand page slightly will not appear on the right-hand page.
Let me know if you have any further questions.