Clicking the triangle next to a location, shows the list of items
excluded from that location (the list in the image contains the
excluded System folder as well as the Applications folder).
Clicking Restore All
will remove these items from the excluded
list and add them back to the original location in the Sources.
Restore
can be used to selectively add any excluded items back
to the original location in the Sources.
To restore at once the system folder’s items, you can uncheck
the Exclude the System folder and its associated items from
scan
button.