This setting allows you to create a list of reasons for which a job may be marked as free of charge. This will appear as a dropdown list once created.
In order to set these up, first, go to your system settings. From here, scroll down to the 'Jobs' section. From here click 'Free of charge reasons'.
From here, you can create your reasons, edit or delete them if need be and change the order in which they would appear in the dropdown menus.