Subscription
A subscription is the collection of all of a customer's shopping cart transactions that are scheduled to be processed on a future date. Subscriptions are listed in your store's Administration Area at: [Orders > Search Subscription Queue].

Subscription Item
A subscription item is a record within a subscription that contains the details for a scheduled transaction for an individual product. If a customer has a subscription with multiple different products, the customer's subscription will contain a subscription item for each of those different products.

Subscription Product
A subscription product is a product with a rule assigned to it to create a subscription for the customer when the product is purchased in the store. Products, including subscription products, are listed in your store's Administration Area at: [Products > Search Products].

Order
An order is a shopping cart transaction that has been placed in the past. Orders are listed in your store's Administration Area at: [Orders > Search Orders].

When a subscription product is purchased, an order is created and a subscription item record is also created. If the customer already has a subscription, the new subscription item will be added to the customer's existing subscription. If the customer does not yet have a subscription, then a new subscription will be created for the customer, and the new subscription item record will be added to the new subscription.

When a subscription is processed, a new order is created, and the subscription item records in the subscription are updated, for instance setting the next Bill Date.