The MTI is the first thing you read on any message. Type four digits and it gets pulled apart below.
| MTI | Meaning | Notes |
|---|
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.