Use Remove from Selection with any select function to remove the new selection from the currently selected stitches.
Remove from a Selection
Place an embroidery on the work area.
Use Box Select
(or another selection tool) to select some stitches.
Click Remove From Selection
.
Click Box Select
and select some of the stitches within your selection.
The new selection is removed from your original selection.
Shortcuts
Select, Home Tab:
Keys: Alt, H, RF