I was just in my project config and noticed a log message stating that the build did not find a "What To Test" file.  I've also seen the TestFlight header "What to test"... this is smelling like a feature - how do I implement/configure this?

Last time I tried to figure this out I couldn't find any info and so postponed thinking about it.  But this time around I found Apple had a document and instructions on making this work.  It was hard to search for and find the article on Apple Docs - so save this link.

Including notes for testers with a beta release of your app

The trick to configure is to create a TestFlight folder/group in your Xcode project.  Inside that special folder create the plan text file with the special name:  WhatToTest.en-US.txt 

This file is then displayed to the testers using TestFlight App.  It can obviously be localized to various languages (note:  en-US in this example).

The article gives an example of a CI script to pull Git log messages and insert them into the WhatToTest file.

Don't Panic