An MT103 is a standardized receipt the SWIFT payment message uses to identify cross-border payments in the form of international wires.
MT103s are the globally-accepted proof of payment associated with international wires (the SWIFT network). They include all payment details such as date, amount, currency, sender, and recipient. MT103s are also great for tracing payments that are missing or delayed because they show the route of the payment between the banks.
This is an exclusive feature for payments going over the SWIFT network (international wires). In contrast, payments made over a country's local payment rails do not come with an MT103.