What are ARN/RNN codes?

Acquirer’s Reference Number (ARN): Also referred to as a Trace ID, this is a unique 23-digit identification number. ARN codes are linked to online VISA and MasterCard debit and credit card transactions between a merchant’s bank (the acquiring bank) and a cardholder’s bank (the issuing bank). The ARN is used to track the transaction’s movement, allowing card brands, card-issuing banks, and processors to locate the transaction and confirm its handling.
Retrieval Reference Number (RRN): A key to uniquely identify a card transaction based on the ISO 8583 standard. The RRN contains a number used with other data elements to identify and track all messages related to a given cardholder transaction (known as a transaction set). It’s usually assigned by the acquirer, but it may be assigned by a merchant or an individual electronic terminal. It can also be used by an issuer to track the transaction.