In my android phone (Samsung S6 6.0.1), there is no real impact for the Settings->DataUsage->RestrictNetworks option.
To me the emails or app updates are always downloaded irrespective of the wifi nw being marked as "restricted" or not.
Is this expected? What is really the use of this "RestrictedNetworks" option?
On Moto X Play (6.0.1), Data usage →WiFi → Network Restrictions displays this self explanatory note - I assume this should be the case with your device also, even if not explicitly mentioned
From Google Support
If you use Wi-Fi, you can set your device to restrict data usage on Wi-Fi networks that are mobile hotspots. This will help you optimize your data usage by preventing apps from using background data or large downloads on these networks.
Edit: To clarify reference made to data network in Moto definition, this helps -
From 8 ways to tame your data-hogging Android phone
Once you designate a hotspot's Wi-Fi signal as a "metered" connection, your Android device will obey the rules of the cellular-data road whenever it's connected to that particular connection—meaning, for example, Android won't exceed any mobile-data limits you've set, or allow your apps to use background data if you've restricted it.
Verified by setting 50 MB limit on mobile data, setting my WiFi as metered and trying to download a 600 MB ROM. I got a warning on my notification bar as shown below
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