Accounts management
Customer data hangs off a strict hierarchy. Each level owns one concern, so you always know where a piece of configuration belongs. The seller sells, the customer is the legal entity, the customer account holds payment, and the billing account produces invoices.
Account hierarchy
The two operational levels: the customer account is where payment methods and mandates live, and the billing account is where the billing cycle is assigned and invoices are produced. A single customer can have several billing accounts on different cycles.