Whenever I use a certain card, the approval code always ends in W (used to always end in Z). Use another one and it always ends in P. Another one ends in D, and another ends in R. Some cards don't end in a letter at all. Does anyone know what letters at the end of approval codes represent?
Auth codes are generated by the issuing FI. If you consistently see the same character at the end of an auth code for a given card, it's likely the issuer just adds that for the 6th character on all transactions.
I cannot find a list of all of them. I believe it is the payment network that adds them.
The list would be 00000 through ZZZZZZ. Again, the number is generated by the issuing financial institution (i.e. if you are using a Chase Freedom card the number is generated by Chase when they approve a charge) along with the response code and other identifying information such as card type and (with the exception of AmEx) Interchange classification.
The payment network provides a Transaction ID, not an approval code.
I can't find a list of the interchange classifications with their designated letters.