What does "Payment method: Credit memo" mean?

If your ticket was exchanged for another ticket that you had already purchased, then our system uses the payment from the initial purchase as payment for the new tickets in the exchange. We apply the previously paid amount as a 'Credit memo' for payment for the new order.

This is what the ticket confirmation email will show when an order was paid for by a previous purchase. In these cases, the previous order was refunded and that refund credit in applied towards the cost of the new tickets.