app-ads.txt is the IAB Tech Lab standard for mobile apps, an extension of ads.txt. The file lives at the root of the developer website named in the app’s store listing, for example https://example.com/app-ads.txt. Each record names an ad system, the seller’s account ID with that system, whether the relationship is DIRECT or RESELLER, and usually a certification authority ID.
Ad platforms and buyers crawl the file. If your network or mediation partner is missing from it, some buyers will skip your inventory, so a stale file costs revenue quietly.
The list shows who is authorised to sell, not who is actually winning impressions. Lists grow over time and are rarely pruned.
See which networks leading publishers authorise in the app-ads.txt reference, and follow the steps in how to check your own file.