Single-message system
Card payments where one message both approves the purchase and posts it, with no clearing file following along later.
The model comes from ATMs. Once a cash machine has handed out the notes there is nothing left to adjust, so it made sense for the same message that asked for permission to move the money too. The US PIN debit networks such as STAR, NYCE and Pulse grew out of ATM networks and inherited the design, as did Canada's Interac. Visa runs its own system by the same name, SMS, alongside the classic dual-message flow. In ISO 8583 the difference shows up in the message type itself: 0200 is a financial request, 0100 merely an authorization.
Simplicity costs flexibility. Restaurant tips, hotel holds and fuel pumps that pre-authorize a flat amount fit poorly when the amount is booked the instant the card is approved. So both models live on side by side, often on the same card.