The release track to which the given APK version(s) will be re-assigned.
This can be one of:
For production releases, you have the choice of rolling the version(s) out
to all existing users simultaneously, or doing a staged rollout so that only
a fraction of your existing userbase will be able to download the new version.
For more information on alpha and beta testing, or staged rollouts, see the
Google Play documentation:
https://support.google.com/googleplay/android-developer/answer/3131213
This field supports substituting environment variables in the form
${SOME_VARIABLE} or $SOME_VARIABLE at build time.