A declined transaction is a payment request that has been rejected by the cardholder's bank for some reason. Transactions may also be declined by the merchant's own payment service provider, if additional fraud prevention rules have been activated.
Declined transactions include a reason code and a reason message in the API response. ISO 8583 offers a standardised format for reason codes, but this standard is not used by all banks. Where an issuing bank is using its own list of reason codes, APEXX will map the code internally and send the ISO code in the API response instead. A list of the ISO reason codes used by APEXX can be found here.