The release track to which the given file(s) will be assigned.

This can be the name of a custom track, or one of the built-in tracks:

For each release track, you have the choice of rolling your new version 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 using the internal, alpha, beta or custom testing tracks, 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.