The first developer preview of Android O already allows you to use Notification Channels, a new feature enabling users to browse and manage notification by categories, Android Police uncovered. While third-party developers need to add support for the new functionality to their apps, Google already updated the Android System service with the necessary code as notifications sent by Android System can now be browsed and managed by categories. Android System notifications pertain to those users receive regarding system-critical information like the availability of over-the-air (OTA) updates and individual requests for permission to install apps from unknown sources. Additionally, Android System in Android O is also sending notifications when an app activates a screen overlay, meaning you'd be notified after doing something like launching a game through Samsung's Game Launcher. This is how Android System's support for Notification Channels was discovered, as seen in the screenshots below, courtesy of Android Police.
In case an app like Twilight starts drawing on a screen, Android System will send you a single notification telling you about what's happening and showing you how many categories of notifications sent by the service are waiting for you in the notification bar. Long-pressing that notification will unveil an "All Categories" field in the bottom right of the notification card, and tapping the newly uncovered option will open Android System's settings page where you can enable and disable its notifications by channels or simply use the global setting supported by older iterations of Google's operating system. Apart from turning Notification Channels on and off, you can also customize the way Android handles them, i.e. allow only specific types of notifications to pop up on your screen or make noise, do both, or do neither and only appear silently in the notification panel.
Overall, Notification Channels allow users more control over not only the types of notifications they're receiving but also the manner in which they're receiving them, which adds yet another layer of customizability to an already extremely customizable operating system. As future developer previews of Android O roll out, third-party developers will likely start updating their apps to support this new feature. More details regarding the next major iteration of Google's operating system are expected to follow next month once Google I/O 2017 kicks off.