Which exact KM are you using?
I am not sure but Warn may be something like degraded performance or network delay. (something I can think of... many be wrong )
httpstatus is one of the consumers of httpMonitor attribute.
The httpMonitor parameter performs client connections to an HTTP-based proxy server and populates the following consumer parameters: httpDownTime httpResponseTime httpStatus. If the httpMonitor parameter encounters an error or warning, the textual description of the problem is set as the parameter’s value. The parameter then issues an alarm or warning for the duration of the problem.
httpstatus monitors the configured httpserver for availability and depending on the threshold configured, it raises Warning (default range 1-1) or Alarm (default range 2-2) if httpserver becomes unavailable.
You can also refer below link from Knowledge Base,
For more details, refer parameter reference for Internet Server Manager KM from Parameter Reference Database link (PATROL Internet Server Manager),
for each monitred page you can set list of return codes for each status level (it can be done in url definition window)