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how, then, would one go about having the entire app act as a subscription service?
Currently, there is no subscription service that could be used for the entire app. A possible way is that you could set your app as free but lock some features in your app. If the user subscribes to the subscription add-on in your app, then the app will unlock the rest of the features.
Here is the document for subscription add-ons: Enable subscription add-ons for your app
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