The Data saver switch allows you to restrict the amount of data apps can send or transmit in the background (when not in use). Some apps can also take advantage of this setting to modify their data consumption while you are using the app, such as by waiting until you tap on an image to load it.
This subsection allows you to configure certain apps that you do not wish to restrict when Data saver is enabled.
This will open a list of apps on your device, allowing you to add each one to the unrestricted list individually.
The selected apps may use data in the background, and will not try to restrict their data usage while in use even if they implement that feature.
System apps can be shown or hidden in this list using the "Show system" setting located in the overflow menu.