Header
Application Data
Message Type Indicator (MTI)
4 digits numeric
BIT MAP (which data elements are present)
ISO Data Element (fields of message, as indicated by BIT MAP)
Trailer
4 digits, classifies high level function of the message
[version] [message class] [message sub-class] [transaction originator]
0 (1987 version)
1 (1993 version)
https://www.codeproject.com/Articles/100084/Introduction-to-ISO
Define the general function of this message (what category)
Define the sub-class / intention of the message
0 REQUEST / require approval
1 response to REQUEST
2 ADVICE action ALREADY TAKEN, no requiring approval, still requiring a response
3 response to ADVICE
4 notification
5-9 reserved
0 Acquirer
1 Acquirer / repeat
2 Issuer
3 Issuer / repeat
4 Other
5 Other repeat