This specifies, as a log2 value, how many bits each transmitted symbol encodes for this single-carrier line's modulation constellation, directly determining how much data rate you get per symbol at a given signal quality. A higher constellation size packs more bits per symbol for higher throughput but requires a cleaner, higher-SNR line to be decoded reliably, while a lower constellation size sacrifices some throughput for more noise tolerance. A line with marginal SNR margin would typically need this configured lower than a clean, short loop, trading maximum speed for reliable operation given that line's actual noise conditions.