This reports the highest Differential Group Delay spike measured on this wavelength during the bin. Because this reading comes from a closed, completed 24-hour history bin for this wavelength/line, PMD is a statistically fluctuating, temperature- and stress-sensitive effect, so this max captures the worst momentary alignment this channel saw -- the instant most likely to have caused a burst of bit errors even if the average looked fine. For example, a max DGD reading of 40 ps on this OCh coinciding with a burst of coding violations logged around the same time is the kind of correlation that confirms PMD, not equipment, caused the errors, across that completed 24-hour day.