1 race on record
How many athletes a mat read, out of how many we can prove went past it. An athlete counts as proven only if a later split recorded them, or they finished. Somebody who vanished from the results entirely is never counted against a mat, because we cannot show they were ever there. That makes every number here a floor on real losses, never an overstatement: the true miss count can be higher than what is shown, never lower.
One physical timing unit, identified by the box id the timing system assigns it. A box is what you drive out to and fix, which is why these pages group by box rather than by mat. One box commonly covers several splits at once (an aid station read on the way out and again on the way back, or a mat shared by two distances), so when it fails, it fails in several places simultaneously.
Each chip is one race, oldest on the left, so a row reads left to right as time passing. Boxes are ranked by their worst race rather than their average, on purpose. A box that is fine four times and dead once is a box that fails, and averaging that against its good days reports it as merely below par, which is the reading that leaves a broken unit in the equipment truck. Check Last raced before acting: a bad record with no race in two years is a retired box, not a current problem.
Each one names a different repair, which is the whole reason they are separate rather than one "unhealthy" label.
A verdict always describes a box's worst mat, not its average across mats. A box reading 99% outbound and 10% inbound has a serious problem at one of them, and averaging to 55% would describe a box that does not exist.