In App Service (Web Apps, API Apps, or Mobile Apps), an app always runs in an App Service plan. In addition, Azure Functions also has the option of running in an App Service plan. An App Service plan defines a set of compute resources for a web app to run. These compute resources are analogous to the server farm in conventional web hosting. One or more apps can be configured to run on the same computing resources (or in the same App Service plan).
You can move an app to another App Service plan, as long as the source plan and the target plan are in the same resource group and geographical region.
The region in which your app runs is the region of the App Service plan. However, you cannot change the region of an App Service plan.
Azure deploys each new App Service plan into a deployment unit, internally called a webspace. Each region can have many webspaces, but your app can only move between plans that are created in the same webspace. An App Service Environment is an isolated webspace, so apps can be moved between plans in the same App Service Environment, but not between plans in different App Service Environments.