Supply chain · also called Authorized Digital Sellers
ads.txt
ads.txt (Authorized Digital Sellers) is a public text file on a website's domain listing which ad sellers and accounts are authorised to sell that website's ad inventory.
ads.txt lets a publisher say, publicly and in one place, "these are the only companies allowed to sell my ads". It is a plain text file at the root of the domain, for example example.com/ads.txt. Each line names an ad system (such as an SSP's domain), the publisher's account ID in that system, whether the relationship is DIRECT or RESELLER, and optionally a certification authority ID.
The IAB Tech Lab launched ads.txt in 2017 to fight domain spoofing: fraudsters were selling fake inventory labelled as famous sites. Now a DSP can compare the domain and seller account in a bid request against the publisher's ads.txt, and refuse to bid if the seller is not listed. Mobile and CTV apps use the equivalent app-ads.txt.
ads.txt is necessary but not sufficient. It tells you a seller is authorised, not that the traffic is human. Publishers sometimes list far too many resellers, which makes the file less useful, and fraudsters have exploited sloppy or outdated entries. It works best combined with sellers.json and the supplyChain object.
Think of it like this
ads.txt is like a list on a band's official website of the only ticket outlets authorised to sell their concert tickets: anyone else selling is suspect.
An example
A line in a news site's ads.txt: "exampleSSP.com, 12345, DIRECT, abc123". A bid request claiming that site via account 99999 on a different SSP is not listed, so the DSP declines.
Related terms
app-ads.txt
app-ads.txt is the version of ads.txt for mobile and CTV apps, a file on the app developer's website that lists which sellers are authorised to sell the app's ad inventory.
sellers.json
sellers.json is a public file in which SSPs and exchanges list their seller accounts, naming who owns each one and whether it is a publisher or an intermediary.
SupplyChain object (schain)
The SupplyChain object (schain) is a field in OpenRTB bid requests that lists, in order, every seller or intermediary that handled the impression on its way from the publisher to the buyer.
Domain spoofing
Domain spoofing is misrepresenting the website where an ad will run, so that inventory from a low-quality or fake site is sold to advertisers as if it came from a premium publisher.
Direct seller
A direct seller is an ad system account that the publisher itself controls and gets paid through directly, marked DIRECT in the publisher's ads.txt file.
Sources: IAB Tech Lab: ads.txt