
Explanation
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Box 1: The Default Provider Subscription
A default Microsoft SQL Server instance will host the database of the App Service resource provider.
In Azure Stack Hub Subscriptions, select the Default Provider Subscription. Azure App Service on Azure Stack Hub must be deployed in the Default Provider Subscription.
Box 2:
Enter the SQL Server details for the server instance used to host the App Service resource provider database and then select Next. The installer validates the SQL connection properties.
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Reference:
https://learn.microsoft.com/en-us/azure-stack/operator/azure-stack-app-service-deploy