This command can also be executed from the SI Editor's Toolbar or by using the keyboard shortcut Alt+X.
The Marks view option shows or hides non-printing characters such as paragraph marks and spaces within the Editing window. Although these marks are never printed, it can be helpful to view them while editing.
To avoid unintentionally inserting ADD and DEL tags when cleaning up formatting by removing extra spaces or paragraph marks, disable the Revisions 
feature temporarily. Once finished, remember to re-enable Revisions.