Reference (from the Latin reference for ‘to trace something back’, ‘to refer to something’)
The internal reference is a certain information/sequence of numbers/internals that you assign internally to find/sort/order orders.
The internal reference does not appear on the invoice or other printouts.
The customer reference is a number or order number of the customer, e.g. an order number or a number that the customer wants to see on the invoice or similar
The customer reference appears on the invoice and on other printouts.