The SDK App Debugger, is a way for developers to see unfiltered TestFlight SDK data coming from an application, before it is appropriated to its section on the build's page. Click on an application to get access to the SDK Debugger where SDK session and checkpoint data will update in realtime.
From here you can pause or clear the data that is appearing.
NOTE: SDK data will show up here from builds that are on TestFlight and those that are not, but have the TF SDK implemented in them. If data is coming from a build uploaded to TestFlight, it will have a checkmark next to the data.
To learn more about the TestFlight SDK, Please see the article here
To learn more about Apps, please see the article here