Invert Selection is only available on the Paint tab.
When Invert Selection is clicked all the currently selected areas become deselected, and the deselected areas become selected.
This is useful when you want to apply an effect to everything except a small area.
If the wrong area is selected, click in the active window, but outside the selected area, to deselect.
Shortcuts
Paint Tab, Select:
Keys: Alt, P, SV
Exclude a Small Area when Selecting
Use one of the selection tools to select the area that you want to exclude from an effect.
Click Invert Selection
.
The previously selected area becomes deselected, and the deselected area becomes selected.