ISO 8583 is an international standard for financial transaction card originated interchange messaging.
It is the International Organization for Standardization standard for systems that exchange electronic transactions initiated by cardholders using payment cards.

ISO 8583 defines a message format and a communication flow so that different systems can exchange these transaction requests and responses. The vast majority of transactions made when a customer uses a card to make a payment in a store (EFTPOS) use ISO 8583 at some point in the communication chain, as do transactions made at ATMs. In particular, the Mastercard, Visa and Verve networks base their authorization communications on the ISO 8583 standard, as do many other institutions and networks.

The ISO 8583 specification has three parts:
Part 1: Messages, data elements, and code values[1]
Part 2: Application and registration procedures for Institution Identification Codes (IIC)[2]
Part 3: Maintenance procedures for the aforementioned messages, data elements and code values[3]
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Message format
A card-based trans­ac­tion typ­i­cally trav­els from a trans­ac­tion-ac­quir­ing de­vice, such as a point-of-sale ter­mi­nal or an au­to­mated teller ma­chine (ATM), through a se­ries of net­works, to a card is­su­ing sys­tem for au­tho­riza­tion against the card holder’s ac­count. The trans­ac­tion data con­tains in­for­ma­tion de­rived from the card (e.g., the card num­ber or card holder de­tails), the ter­mi­nal (e.g., the ter­mi­nal num­ber, the mer­chant num­ber), the trans­ac­tion (e.g., the amount), to­gether with other data which may be gen­er­ated dy­nam­i­cally or added by in­ter­ven­ing sys­tems. Based on this in­for­ma­tion, the card is­su­ing sys­tem will ei­ther au­tho­rize or de­cline the trans­ac­tion and gen­er­ate a re­sponse mes­sage which must be de­liv­ered back to the ter­mi­nal within a pre­de­fined time pe­riod.
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An ISO 8583 mes­sage is made of the fol­low­ing parts:

Message type indicator (MTI)
One or more bitmaps, indicating which data elements are present. It consists of primary bitmap and secondary bitmap. The first bit of the primary bitmap indicates whether the secondary bitmap is present or not.
Data elements, the actual information fields of the message
The place­ments of fields in dif­fer­ent ver­sions of the stan­dard varies; for ex­am­ple, the cur­rency el­e­ments of the 1987 and 1993 ver­sions of the stan­dard are no longer used in the 2003 ver­sion, which holds cur­rency as a sub-el­e­ment of any fi­nan­cial amount el­e­ment. As of June 2017, how­ever ISO 8583:2003 has yet to achieve wide ac­cep­tance. ISO 8583 mes­sag­ing has no rout­ing in­for­ma­tion, so is some­times used with a TPDU header.

Card­holder-orig­i­nated trans­ac­tions in­clude pur­chase, with­drawal, de­posit, re­fund, re­ver­sal, bal­ance in­quiry, pay­ments and in­ter-ac­count trans­fers. ISO 8583 also de­fines sys­tem-to-sys­tem mes­sages for se­cure key ex­changes, rec­on­cil­i­a­tion of to­tals, and other ad­min­is­tra­tive pur­poses.

Message type indicator (MTI)
The mes­sage type in­di­ca­tor is a four-digit nu­meric field which in­di­cates the over­all func­tion of the mes­sage. A mes­sage type in­di­ca­tor in­cludes the ISO 8583 ver­sion, the Mes­sage Class, the Mes­sage Func­tion and the Mes­sage Ori­gin, as de­scribed below.

ISO 8583 version
The first digit of the MTI in­di­cates the ISO 8583 ver­sion in which the mes­sage is en­coded.

Message class

Po­si­tion two of the MTI spec­i­fies the over­all pur­pose of the mes­sage
Message class
Po­si­tion two of the MTI spec­i­fies the over­all pur­pose of the mes­sage