Thanks for kindly response, Charles.
Customer does monitoring the critical event in log files in all of servers they have.
What they do is that in case of occuring Severe error in specific server, their maintanence team investigate server/service status and do troubleshoot the error.
Seems like other solutions/products which they have have provided description of critical level error.
That's why they are asking if we have documentation of description including Severe error.
It would be much better if we have those kinds of documentation.