The Transaction entities are debts and payments.
When an order is created, a "debt" transaction is also made for the value of that order.
Payments done by or expected from the customer become "payment" transactions which can have a status of paid or unpaid.
An order is paid when the total value of all payment transactions equals the total value of all debt transactions for that order.
Do not confuse with SUB/SubscriptionTransaction, which are payments with credits.