Before, Android’s Data Saver would shut down all data usage when a specific limit was hit on the system’s data usage statistics. But, in Android 7.0 the feature will be more nuanced and smarter. Now, Android will check to see if you’re on a metered wireless connection (including prepaid and roaming) or on an unlimited plan, and if it finds you’re on a metered plan it will continuously limit the amount of background data an app can use. Of course, you can always whitelist certain apps if you want to have them be able to update to the fullest in the background.
The aim is to help make sure you can avoid (or at least delay as much as possible) hitting your data usage limit in the first place, rather than only taking action once your limit has been hit. The new features have been discovered by changes noticed in the “ConnectivityManager” API in the Android Nougat SDK.