test.nextdns.io documentation

Hi folks,

I am trying to understand the output of test.nextdns.io and then specially the IP addresses listed.

Client IP is equal to my WAN, where Source IP is the ip address of my VPN server (which should be used to route DSN traffic).

I am using the DNS Shield feature in my UDM SE, so not the CLI (for sake of future resilience). 

Can someone help with explaining the difference between Client and Src IP?