application protocol data unit (OSI) (APDU)

Application Protocol Data Unit (APDU) is a protocol data unit of the application layer. APDU is the data passed to or received from the lower layer for transport to the corresponding layer.

APDUs are embedded in the Transport Protocol Data Unit( TPDU) in the transport protocol and exchanged between the smart card and the card reader in smart card applications. APDUs are complex data structures consisting of protocol control information and, where applicable, user data, which are passed to the presentation layer as service data units( PSDU).

In the electronic exchange of business data, for example, APDUs must represent the contents of an invoice in an interpretable form and contain information about all recipients and the number of copies. Like user data, protocol data units are exchanged only in the data transfer phase.