This reports the lowest received optical power sampled for this wavelength during the bin. Because this reading comes from the still-filling, live 15-minute bin for this wavelength/line, a deep minimum captured here -- even if the average looks fine -- is how you catch a brief power dip, such as a protection switch or a micro-bend event, that an averaged figure would otherwise smooth away. For example, an average of -12.5 dBm but a recorded minimum of -18.0 dBm in this same bin means the channel briefly dropped 5.5 dB at some point in the interval, so far within this still-open 15-minute interval.