The percentage of existing production users to which the given version code(s)
should be rolled out.
Google Play limits the percentage values to:
If you enter no value here, or 100%, the app will be rolled out to all users.
This field is ignored if you choose a release track other than 'production';
releases to other tracks are always rolled out to 100% of the existing users.
For more information on 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.