HTTP referrer allows you to obtain information about the source (URL) of your survey. This information is saved and stored in individual reports. It allows to match individual report with survey source (eg. website on which survey link has been placed).
Referrer allows you to :
NOTE. HTTP referrer works automatically for all online surveys and tests and doesn't require any configuration.
You can read more about referrer on Wikipedia.
Functionality is available for Professional and Enterprise plan.