Apple has created two extension types for notifications that you can use to take the notification experience up a notch from custom actions. The available extensions are service extensions and content extensions. These extensions are both very powerful and relatively simple to implement. In this section, you will learn how to add both a service extension and a content extension to the Notifications app.
Service extensions are intended to act as middleware for push notifications. A service extension receives a notification before it's displayed to the user. This allows you to manipulate or enrich the notification's content before it's shown to the user.
A service extension is perfect if you're implementing end-to-end encryption, for example. Another great use for a service extension is to download a media attachment from a push notification, save it locally, and add it as a media attachment to the notification contents...