I wrote some parts of Player that actually return NACK in certain
circumstances. NACK is the oposite of ACK (acknowledge). When a client
or a driver sends a message to another driver - it is not certain that
the other driver can accept that message (it happens that given message
type is not supported by the driver) - in such a case NACK is replied.
Or, when you send request to a driver and it cannot be completed, NACK
is also replied.

Thanks ! :-) ..... and I used to think that it is some abbreviation !