Use Remove from Selection to remove the new selection from the currently selected area.
Remove an area from a Selection
Place an image on the Canvas.
Use Box Select (or another selection tool) to select some an area.
Click Remove From Selection .
Using Box Select , select an area within your selection.
The new selection is removed from your original selection.
Shortcuts
Draw Tab, Select; Paint Tab, Select:
Keys: Alt, D, SM; Alt, P, SM