To help customers better understand invoices we could provide some kind of reference on line item basis they can refer to. As a result each invoiced line item can is referenced by a customer order/reference.
This will be handled by FinancialForce with invoice templates to show e.g. the purchase order number. See https://docs.google.com/document/d/1N4SY14KXzA4-J7Zbjr4EB0RREbdLDRc0GyX4yxbDiNw/edit.
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