Referral Source targeting lets you target campaigns to visitors arriving from social media (Facebook, Twitter, Instagram) or affiliated websites (voucher code sites, comparison sites).
In the platform, Referral Source URL looks the same as URL targeting – the main difference is that you only need to type in part of the referrer website’s URL that includes information that says where the visitor has come from.
That information can be:
the referrer’s domain name
the first few characters of their URL
their complete URL
Referral Source targeting cannot be used to target visitors arriving from search, email, PPC or banner ads as mediums like AdSense and Gmail do not send this referral information to your site. To get around this, try using URL or page history targeting to look for any idenitifiers passed by these mediums i.e. utm_campaign or gclid.