This is the target signal-to-noise margin, in tenths of a dB, that the VDSL modem must achieve, at a bit error rate no worse than 10-7, during its training sequence for initialization to succeed at all, directly gating whether the line can even come up. Setting this too high on a marginal, long, or noisy loop can cause repeated initialization failures, since the modem simply can't reach that much margin at any workable rate; setting it too low sacrifices error resilience once the line is up. A provider deploying to known long loops might deliberately lower this target from a default 6 dB to 3 dB specifically to get lines with borderline reach to initialize successfully, accepting somewhat reduced noise headroom in exchange.