Use Remove from Selection to remove the new selection from the currently selected objects.
Remove from a Selection
Place several objects on the work area.
Use Box Select
(or another of the selection tools) to select some of these objects.
With Remove From Selection
selected, click Box Select
and select some of the objects within your selection.
The new selection is removed from your old selection.
Shortcuts
Home Tab, Select:
Keys: Alt, H, RF