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A three-layer model is used to compare payments schemes.

Policy: The semantics of the payment scheme. This includes refunds policies, and the liabilities incurred by customers, merchants and financial institutions.

Data flow: The requirements for storage of data by and communications between the parties. This includes not only the data flows for payments themselves but also for refunds, account enquiries and settlement.

Mechanism: The methods by which the necessary security requirements for messages and stored data are achieved.

All three abstraction levels are tightly coupled since policy makes requirements of data flow and data flow makes requirements of mechanism.