When you try to make a refund for an online payment, the refund is pro-rated to each invoice item. The option to make refund to a particular line item of an invoice is not available yet, but would be very useful to have in order to get your accounting data right and be able to understand better what items are being refunded to your customers.
It can be used to refund a Garantee Deposit or any optional feature to the same line item of the incoice, so that you know and track perfectly what has been refunded.