Principles of istio Locality Load Balancing

I learn from envoy’s offical doc,envoy support Locality Load Balancing;and in istio offical doc,I see istio use Region/ Zone/ Sub-zone to realize Locality Load Balancing.

I Istio pilot source, I see code use to label region info

Can anyone tell me,how these componet(pilot/istio/envoy) togther to realize Locality Load Balancing