What is a status word?

Julien SIMON

The status word is a 2-byte value, returned to the reader after an application has processed an APDU. Data is optional in a response APDU, but the status word is mandatory.

If the process() function runs to completion, status word 0x9000 is implicitly returned (meaning everything is OK). If the application needs to return another value (probably to signal an error condition), it can do so by invoking ISOException.throwIt(short value)

javacard.framework.ISO7816 defines a number of frequently used values, but you’re free to use anything you’d like, provided that it’s not already used by the platform itself.