To further elaborate An app service always runs within an App Service plan and an App Service plan defines a specific set of compute resources for a web app to operate.
When creating an App Service plan in a chosen region (e.g., West Europe), a corresponding set of compute resources is established in that region. Any apps added to this App Service plan will run on these compute resources, as specified by the App Service plan configuration.
Further in the dedicated compute tiers (Basic, Standard, Premium, PremiumV2, PremiumV3), the App Service plan defines the number of VM instances the apps are scaled to, so each VM instance in the App Service plan is charged. These VM instances are charged the same regardless of how many apps are running on them.
To avoid unexpected charges, see Clean up an App Service plan.
You may refer to this detail document on Azure App Service plan overview
Also region-specific pricing information, see App Service Pricing.
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