When a financial transaction takes place it often has a reference number to the sale (aka. an order).
This flow describes an implementation where Billing is handled outside of Salesforce (eg. QuickBooks, SAP, Stripe, Authorize.Net, NetSuite, etc). The billing system will call the Salesforce API to create or update the Payment Transaction record.
Inbound
Outbound
Internal to Salesforce
This flow describes an implementation where Billing is handled within of Salesforce (eg. AccountingSeed, FinancialForce, ZoomZoom Financials, etc). Salesforce automation can be put in place to create or update the Payment Transaction record in Salesforce.