Whenever you post a link, you can choose to add a special query parameter to the link. When the ?source=<value> query parameter is present, Ackee will treat it as a referrer.
For example, here's what you can do when you want to send a newsletter to your subscribers. If you link to your site with example.com, anyone who clicks on it wouldn't show up under "Referrers".
But if you link to example.com?source=Newsletter anyone who clicks on that link will show "Newsletter" as the referrer source. This will allow you to see how many people have clicked on your link in the newsletter.
The Referer request header contains the address of the page making the request. It's not available in all situations (e.g. when opening a link from an application). Ackee will use and show the header in the UI when no source parameter is available.