When selecting filtering options, after pressing the down arrow on the right of the column header, a menu comes up with various filtering options and the list of values available in the column.
Sometimes there are many values I can choose from, so selecting those few I need may take time. Often I want to see the opposite, I mean so far I looked at for those few selected, but now I want to have a look at all others. This requires that I invert the selection: everything that was selected so far should be unselected and everything that was not selected so far should now be selected.
So a selection option called "Invert selection" would be useful. Clicking this would flag all unflagged values and would clear the flag from all flagged ones.