What advertisers need to know about sourced traffic

A number of advertisers, concerned with the lack of transparency in digital advertising, are now tending to play safe and deal direct with trusted, ‘premium publishers.’
These advertisers are competing for the highest quality ad inventory on the most reputable digital properties, willing to pay a premium to try to keep their brands safe as well as attempting to avoid ad fraud.
The unseen/unknown hitch
What exactly is 'sourced traffic'? It occurs when digital media publishers/vendors/