What does "CP NACK" mean?

Hello,

I have a log entry stating "session closed CP NACK" with these packet counters "0(0) 0(0) 1".

All I found was an explanation for the log entries, and that only contains a table:

CP NACK response received

But more details are missing.

Can you tell me in which documentation I can find details about this "CP NACK"?

Thanks in advance!