Traffic is classified as Referral when it comes from websites that are not social networks. For example, other blogs or affiliates who link to you.
Google Analytics filters traffic to this channel when:
- Medium exactly matches referral.
Unrecognized Search Engines
When a user visits your site from a search engine most of the times, Google Analytics recognizes it and classifies it as Organic Search in the Traffic Reports. But if the search engine has not yet been added to the official search engine list, the traffic will be classified as Referral instead.