The MTI is the first thing you read on any message. Type four digits and it gets pulled apart below.
| MTI | Meaning | Notes |
|---|---|---|
0100 | Authorization request | Real-time authorization; holds funds, does not clear |
0110 | Authorization response | Carries the DE39 verdict |
0120 | Authorization advice | A fait accompli; the issuer must accept it |
0130 | Authorization advice response | |
0200 | Financial request | Authorization and capture in one step |
0210 | Financial response | |
0220 | Financial advice | Offline or stand-in transactions sent afterwards |
0230 | Financial advice response | |
0320 | Batch upload advice | |
0400 | Reversal request | Undo the original and wait for an answer |
0410 | Reversal response | In practice this is almost always 00 |
0420 | Reversal advice | Already reversed here; the other side must follow |
0430 | Reversal advice response | |
0500 | Reconciliation request | |
0510 | Reconciliation response | |
0520 | Reconciliation advice | |
0800 | Network management request | Sign-on, sign-off, echo test, key exchange |
0810 | Network management response | |
0820 | Network management advice |
An MTI is four digits and each one carries its own meaning — together they are the message's identity. Digit 1 is the standard version, digit 2 the message class (authorization, financial, reversal, network management…), digit 3 the function (request, response, advice), and digit 4 the originator plus whether this is a repeat.
This digit decides whether the other side is allowed to say no. Function 0 is a request — "I would like to do this, do you agree?" — and can be declined. Function 2 is an advice — "I have already done this, you must follow" — and can only be acknowledged.
So 0400 and 0420 are not synonyms: one asks to reverse, the other announces a reversal that already happened. Getting this wrong means sending 0400, being declined, and having no way back because you already voided the transaction locally.
Odd values in digit 4 mean repeat: 0400 is the first attempt, 0401 a re-send of that same one. The rule for repeats is that nothing but this digit may change — DE11 (STAN) and DE7 (transmission time) especially must keep their original values. A great many "double charge" incidents trace back to a retry that helpfully generated a fresh STAN, which the receiver reads as two separate transactions.
Position 1 of the MTI names which edition of ISO 8583 the message is written in: 0
for the 1987 edition, 1 for 1993. It is easy to read that as history and assume the
higher number replaced the lower one. It did not.
Both are in production at the same time, inside the same card scheme, for the same transaction.
Authorization messages are the 1987 form and begin with 0 — 0100,
0110, 0200, 0400. Clearing messages are the 1993 form and
begin with 1, such as 1240 for a first presentment. So one purchase
produces a 0100 on the authorization leg and a 1240 in the clearing
file, and those two are not different transactions.
The reason to care is that the two editions do not share a field dictionary. The same data element number can hold different content, and content can move between numbers — the merchant category code sits in DE18 in authorization and DE26 in clearing, for instance. A parser configured for one edition will decode the other without complaining and give you plausible nonsense. The crosswalk is in authorization versus clearing.