An IAB Tech Lab standard that lets app publishers declare which ad networks are authorized to sell their inventory.

Where do I put the app-ads.txt file?

At the root of the developer website listed in your App Store / Google Play page, e.g. https://studio.com/app-ads.txt.

What is a Certification Authority ID?

An optional TAG-ID of the ad system, the 4th field of an app-ads.txt record.

With MAX, your app-ads.txt must cover AppLovin and every network you mediate. This tutorial takes you from the MAX dashboard to a verified file.

In the MAX dashboard open Account → IAB Supply Chain Validation (app-ads.txt + sellers.json) and note the lines it shows for your account.
List the networks enabled in your MAX ad units (AdMob, Unity, Mintegral, Meta, ironSource, Liftoff, Pangle…).
In the App TXT generator add AppLovin and each of those networks with your IDs — partner lines come from current templates.
Download the file (or host it) and publish it at https://yourdomain.com/app-ads.txt — the developer website from your store listing.
Come back to the MAX validation page after the next crawl and fix anything still reported as missing.

Keep one file for all apps that share the developer website.
When you add a new network to MAX, add its lines the same day — otherwise its bids may be lost while it can’t verify you.
Use the checker to compare your live file with what the networks expect.

MAX lists the lines it knows about; you still publish the file yourself. A generator saves you from copying hundreds of partner lines by hand.