In the Publishing Hub it is possible to define specific editions as free. This is useful for marketing purposes or when are delivery problems with the printed paper. Simply choose the edition that should be free in the Publication Overview and press on the gift symbol. When you don't want it to be free any longer, just press the icon again to revert it.
Note that this option only applies to iOS and Android apps. In some cases it will also work on web, provided you use the same validation as in the apps.
When you have defined the edition as free, it will look like this: