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.