If you have already created an account, use this call to create a subscription for that account. The call takes an account ID, creates a new subscription, and, optionally, generates an invoice on that account and applies a payment right away.
There are two common scenarios for which you may wish to use this call.
The most common usage is this: One of your customers has created an account for a free trial and now wants to activate that subscription. Use this call to do so and to provide payment information and apply a payment.
Less commonly, you may wish to create a second subscription for an existing account.