The Select menu > Deselect command brings up a submenu with three choices. Those choices are:
Pointer/Window: This command sets Section Designer in its default select mode where you can deselect shapes either by left clicking on them or windowing them.
Intersecting Line: This command sets Section Designer in the intersecting line select mode (or more accurately in this case deselect mode).
All: This command automatically deselects all shapes in the section.