[WCF]Service Identity for authentication against the server-side endpoint
Here are some reference for you who want to get more ideas on what, why and how of the "Service Identity" used in WCF service endpoint for mutual authentication:
#Overriding the Identity of a Service for Authentication
https://msdn.microsoft.com/en-us/library/bb628618.aspx
#Service Identity and Authentication
https://msdn.microsoft.com/en-us/library/ms733130.aspx