Release shines with multi-service applications. Your team probably has a frontend, a backend, a database, some API's and potentially numerous services that have external hostnames, but generally there is one main link into your app.
By selecting a Primary App Link, Release will only show a single hostname url that directs to your main app on the Environment list summary and in GitHub PRs. This can make it much easier to share Ephemeral Environments.
If you need to view other supporting services besides the main primary app, it is still possible to see all of the remaining hostname urls on the Environment details page.
You can locate the Primary App Link feature under Application Settings/Advanced Settings. A list of your services will appear and you can select any one of them, usually the main frontend application. It will follow the typical hostname url pattern that Release uses to generate Ephemeral Environments, which contain a unique randomness variable and the domain variable.