This is the highest value recorded during the current performance-monitoring interval of the differential group delay (DGD) measured on this channel during the current performance-monitoring interval, expressed in picoseconds, reflecting how much polarization-mode dispersion is spreading the two polarization components of the signal apart in time. The highest DGD seen in the interval is the value an admin watches most closely, because a coherent receiver can only tolerate DGD up to a certain limit before its PMD-compensation algorithm can no longer keep the signal readable. For example, if differentialGroupDelayMax on an older buried cable route repeatedly approaches the receiver's PMD tolerance limit, an admin would flag that route as a risk for any future higher-baud-rate upgrade.