Buyers use the file to check whether an account is authorized to sell ads on the site. The IAB Tech Lab specification defines the fields in each record. (Google ads.txt FAQ ↗ · IAB Tech Lab ads.txt specification)
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Reading an ads.txt line
Illustrative line: exchange.example, seller-42, RESELLER, cert-7. The first field identifies the advertising system; seller-42 identifies the authorized selling account on that system. RESELLER describes who controls that account, and cert-7 stands for an optional certification identifier. These are fictional values: the actual account and certification identifiers must come from the partner’s verified instructions.
What DIRECT and RESELLER mean
DIRECT means the publisher controls the named account with that advertising system. RESELLER means another entity controls the account and is authorized to resell the inventory. Match the record to the actual arrangement; do not assume every account supplied by a partner should be labeled DIRECT.
When a public entry needs investigation
If a seller appears in the public file but nobody can identify the partner using it, save the row and check it against the partner’s current instructions. Verify the account before removing it. A forgotten authorization and an active reseller can look alike in a plain text file.