Previously, 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. This all sounds pretty awesome, but we're also hoping for better data usage monitoring that can more accurately determine usage when your plan doesn't count certain apps (as with T-Mobile's Binge On).
source: Android Central