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
beeshyams' answer seems more correct. But on a test with samsung galaxy s6, the "restrict networks" setting has no impact at all though i have applied a mobile data limit in "settings->data usage". Looks to me different behavior in different devices. I am currently talking to Samsung support on this. Will update if I get an answer.
UPDATE: Talked to samsung support. They accessed my phone remotely and they were not able to validate my use case. They also said, they have not received any complaints from users regarding this feature. They recommended taking this phone to samsung service center to check with technician. I don't think I will be the first person testing this feature on samsung s6????? Anyone knows more info about this feature on samsung, please share.