Application ID URI value uniquely identifies the application when it is used as a resource, allowing tokens to be requested that grant access. It is used as a prefix for scopes you create. The App ID URI is one of the ways an application is identified in protocol messages. For a single-tenant application, the App ID URI need only be unique within that tenant. For a multi-tenant application, it must be globally unique so Azure AD can find the app across all tenants. Global uniqueness is enforced by requiring that the App ID URI's host name matches one of the Azure AD tenant's verified publisher domains.
For a single-tenant app, you can use the default value, which is in the form api://<application-client-id>. You can also specify a more readable URI like https://contoso.com/api based on one of the verified domains for your tenant. For a multi-tenant app, you must provide a custom URI.
Thanks,
Prasad Das
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