This read-write integer, in millidecibels within -100000 to 100000 and defaulting to 0, sets an attenuation value subtracted from all RxPower readings on a port, and immediately clears the associated channel and spectrum tables when changed. Admins use this to calibrate power readings against known external attenuation, such as a tap or splitter in the optical path. For example, setting this to 3000 (3 dB) would compensate for a known 3dB loss from an inline splitter before the OSA's measurement point, but the admin should expect the channel and spectrum tables to reset and repopulate afterward.