Availability zones are physically separated collections of datacenters in a given region. Deploying resources across zones ensures that outages that are limited to a zone don't affect the availability of your applications. This architecture shows how you can improve the resiliency of an App Service Environment deployment by deploying it in a zone-redudant architecture. These zones aren't related to proximity. They can map to different physical locations for different subscriptions. The architecture assumes a single-subscription deployment.

learn.microsoft.com/en-us/azure/architecture/web-apps/app-service-environment/architectures/ase-high-availability-deployment