This encodes the line's symbol rate as a profile index S calculated from the actual desired symbol rate SR divided by the base symbol rate BSR of 67.5 kbaud, giving a standardized way to express the transmission rate for single-carrier VDSL rather than specifying raw kbaud directly. It's a low-level physical-layer configuration parameter that determines the fundamental signaling rate of the line, which combined with the constellation size determines the line's raw data rate. Two SCM profiles differing only in this symbol rate value, all else equal, would support proportionally different maximum data rates.