The Referring URL is the HTTP Referrer 
value that impact.com records from the user's browser at the time of a click and is a good indicator of where traffic to your website is coming from. However, it is not 100% trustworthy or reliable as a tracking source, as it is highly dependent on how the click was made or the link was opened. There is a myriad of factors that can affect what gets sent to impact.com as the Referring URL.
If you see clicks in the Performance by Referring Domain report with a “No referring domain" status in the Domain column, it often means that there is no Referring URL information available due to any of the following reasons:
The user entered the site URL manually into the browser address bar.
The user visited the site by a browser-maintained bookmark.
The user visited the site as the first page of that window/tab, like right-clicking and opening a link in a new browser window.
The user clicked a link in an external application.
The user switched from a secure protocol (https) to a non-secure protocol (http).
The user switched from a secure URL (https) to a completely different secure URL (https).
The user may have security software installed (antivirus, firewall, etc,.) which strips the referrer from all requests.
The user is behind a proxy server which strips the referrer from all requests.
The user visited the site programmatically without setting the referrer header (the use of searchbots).
Some browsers’ privacy policy limit access to not allow
HTTP_REFERERto be passed.The user has some browser add-on or extension that blocks it for privacy reasons.
Note: impact.com recommends that you do not use the Referring URL option in payout groups when determining payouts to partners, and to not lean too heavily on the Referring URL as a source of data for decision making.
