"Print Unmatched Unit Flag" is a useful option in case user need to extract matched text out of the original text.
Open Replace Dialogue, there is a "Print unmatched units" checkbox.
If "Print unmatched units" is checked, all the text not matching the pattern will be copied to the output, this is the default setting.
If "Print unmatched units" is unchecked, only the replacement of matching text will be copied to the output.