Terminology used across different subscription billing systems to define what a subscription is.
- https://dev.recurly.com/docs/create-subscription
- https://stripe.com/docs/api/curl#subscriptions
- https://developers.braintreepayments.com/reference/response/subscription
- https://apidocs.chargebee.com/docs/api/subscriptions
- https://reference.chargify.com/v1/subscriptions/subscriptions-intro
- https://www.zuora.com/developer/api-reference/#tag/Subscriptions