What do letters at the end of approval codes represent?

[BijouMan]

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?

[K-in-Boston]

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.

[BijouMan]

I cannot find a list of all of them. I believe it is the payment network that adds them.

[K-in-Boston]

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.

[BijouMan]

I can't find a list of the interchange classifications with their designated letters.