Application protocol datagram unit. These are the frames that are exchanged at application-level between an application running on the computer and a smartcard. The format of those frames is defined by ISO 7816-4 and checked by the system's PC/SC stack. The command (application to card) is called a C-APDU and consists at least of four bytes denoted CLA (class), INS (instruction), P1 and P2 (parameters). The response (card to application) is called a R-APDU and consists at least of two bytes denoted SW1, SW2 (status word). Note that this is a request/response scheme: the smartcard has no way to send something to the application unless the application asks for it.