You can determine how you would like the area not selected to be displayed. You can choose among [Hide], [Check] and [Darken]. [Check] and [Darken] make it easier to see which area is NOT selected.
Edit only applies to the selected area.
If the selected area is not closed such as a straight line, then this option will not make any difference.

More easily, clicking this icon changes this option in a sequential order.

*Hide

*Check

*Darken