This read-write value is the operator-requested attenuation, in millidecibels, that should be applied to the signal received on this optical port. A technician writes to it to deliberately reduce incoming optical power, for example to protect a receiver from overload, and the change is applied and persisted immediately rather than staged for a later commit. Setting it to 3000 mdB (3 dB) attenuates the received signal by 3 decibels; the special value -200000 signals that receive attenuation isn't supported on that port.