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This was extracted from the user mailing list

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Subscription: keeps track of entitlement to SubscriptionResource for a certain period of time (ie history of what was purchased/etc, used to check permission to access, etc)
SubscriptionResource: you would have one of these records for each resource, such as a file or a set of files, or a publication, or whatever
ProductSubscriptionResource: this is where you specify how much time and to what resources the customer is entitled to when they purchase the Product; the system automatically creates a Subscription record for each of these when the Product is purchased
If you want a customer to pay for a long period up front, you would just configure a Product for that. If you want the customer to pay monthly, but sign up for a longer period of time (like a year) then you would have a Product setup for 1 month of access, and create a ShoppingList for a recurring Order to happen once a month for 12 months.

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Another source with more details

Here is an An interesting article about subscription.