You can create multiple refunds for a transaction. If it was paid by Credit Card then the total sum of refunds can not exceed amount of the transaction. For bank-link paid transactions there is no such limit.

See also the State model of refunds.
Note that you should verify state of the refund in the response. You may be returned 200 response (resource created) but it's state may be FAILED.
While the refund is in state SENT it's amount is reserved in shop's account.
You need sufficent balance on the account to make refunds.

If transaction was partially paid by gift card then transaction channel is "Mixed" and the created refund will contain sub-refund link(s) _links > sub you can use to query for additional details

If transaction was partially paid by gift card then transaction channel is “Mixed” and you have Paytrail payment methods enabled you should
a) always provide email for the Paytrail refund
b) use get transaction details to determine if sub-transaction was Paytrail and only provide email if it was