7. The Menus / 7.3 Functions in the View Menu
If the menu entry Mark Extreme Values is turned on, all black and white areas on-screen will be marked. If this option is active, the title bar of document will show "Ex".
By using Options/Display/Extreme Values (see chapter 7.10.1.25) you can define, how the areas will be marked and which tolerance will be used for marking.