It is the result of decoding wire and transport protocols containing the bi-directional data payload.
Wire data is the observed behavior and communication between networked elements which is an important source of information used by IT operations staff to troubleshoot performance issues, create activity baselines, detect anomalous activity, investigate security incidents, and discover IT assets and their dependencies.
In these types of scenarios, wire data is used to measure order transactions for real-time reporting on transaction volume, success, and failure rates; tracking patient admission rates at hospitals; as well as reporting on the weights and measures of airplanes prior to take-off.
However, web servers do not log the transaction payload and so would not be able to show which HTTP status code 200 responses were for pages with a “service unavailable” message.
That information is contained in the wire data or transaction payload and is not necessarily logged by the server.