Synthetic error codes pertain to a combination of the HTTP Response code on a page, sometimes coupled with a custom error code about the way the synthetic measurement ran and errored. The HTTP Response code is 3 digits long, i.e., XXX. Codes that follow the format 2XX show that a page loaded and was available. Codes that follow a 3XX format show that a measurement redirected. Codes that follow a 4XX format show that a page does not exist, possibly due to being linked or typed incorrectly. 5XX codes show that a page is unavailable due to a variety of reasons, but usually because a server or similar is down in the network. 6XX codes are related to an error with a synthetic monitor.
6XX codes are often coupled with our 4-digit custom error codes that tell you what went wrong with the synthetic agent. While there are too many commands that can fail while running a monitor to list out here, the exact information can be found next to the error code for the measurement on the Performance Detail page for Real Browser measurements. The format of the error code with the HTTP Response code in front of it is XXX : XXXX
See also: Synthetic Custom Error Codes