The “Mobile app” setting links your conversion action to a particular app. This lets Google Ads alert you if you have multiple conversion actions tracking app installs for the same app, so that you can avoid counting two conversions—and potentially bidding for two conversions—for a single app install.
To use this setting, simply look up your mobile app by name, 9- or 10-digit Apple ID, or publisher.
Only apps published on iTunes will appear when you search. It may take up to 8 days (though usually under 48 hours) after publishing for your app to appear in the search results.
If your app doesn’t appear when you search, but you want to set up conversion tracking, you can enter the 9- or 10-digit Apple ID in the field and click "Done." The Apple ID is the unique identifier for your app on iTunes. You can find it in the “App Information” section of iTunes Connect.
It’s best to use the conversion action to track only the app that you’ve set in “Mobile app.” If you want to track conversions for multiple apps, create separate conversion actions for each.