Apologies if this is already answered somewhere, but I couldn't see it in the FAQ.

When I click on a flight in Availability/FS, the bottom-left of the window normally displays an "On-time rating", shown as a score out of 5. I assume that 5 = "always on time", and 0 or 1 = "always massively late", but is there any more specific interpretation of these scores anywhere? Does it refer to the frequency of delays, or length of delay when it occurs, or some combination of the two?

Many thanks!