If the transaction is authorized but not captured it will reverse the authorization.
If the transaction is captured but not yet settled it will void the capture.
If the transaction is settled and the merchant has been paid a credit to the card holder is generated and processed.
If the amount refunded is less than the original amount, then an adjustment is made if not settled, or a partial credit is given if settled.
A Refund can also be sent without a transactionID and offsetting payment. In order to allow, special configuration must be set on the merchant record. Because the refund is not tied to any transaction, a credit will be generated to the cardholder and no checking is performed.
N O T E:
For testing set the following fields after selecting the "Try It" button
externalReferenceID = Unique record identifier for processing - you assign Numeric Min = 16 Max 19
transactionDate = Set to UTC Timezone
Link to UTC time = https://time.is/UTC