ads.txt

ads.txt stands for Authorized Digital Sellers. It is a standard from the IAB Tech Lab. A publisher places a plain file at the root of its domain naming each ad system allowed to sell its inventory and the publisher's account ID with that system. Buyers can read the file and refuse offers from anyone not on the list, which makes domain spoofing much harder in display advertising.

Its role in search arbitrage is supporting, not central. Search feeds are not bought through open auctions where a buyer reads ads.txt; they are controlled by contract and by Google's or Microsoft's own site approval. But the file still matters in three ways. AdSense publishers are asked to list their publisher ID in ads.txt, so a correct file is part of a clean set-up. Arbitrage sites that also run display ads need it for those units. And when buying traffic from publishers, a missing or chaotic ads.txt is a quick sign of a poorly run site.

The limit of ads.txt is that it proves who may sell, not that the visitors are real. A perfectly authorised seller can still deliver bot traffic.

Think of it like this

It is a brand's list of authorised dealers on its website. If a shop is not on the list, the goods may be fake, though being on the list does not guarantee good service.