This is the measured signal loss on this VDSL line - the difference between what the peer transceiver transmitted and what this unit actually received - expressed in tenths of a decibel, essentially a real-time measure of how much the copper pair itself is degrading the signal. It's the go-to metric for estimating how long a customer's physical loop is and how much margin the line has for future degradation, since attenuation generally increases with loop length and wire gauge. A line showing 350 (35 dB) of attenuation is on a notably long or poor-quality loop compared to one showing 100 (10 dB), and would be expected to support lower maximum sync rates as a result.