This specifies the length, in octets, of the fast (low-latency, non-interleaved) codeword for a single-carrier VDSL line, with a special value of 0 meaning the line instead uses a single unified latency transport class rather than separate fast and slow channels. It determines whether this SCM line actually has a distinct fast channel available at all for latency-sensitive traffic, or whether all traffic shares one transport class regardless of latency needs. A profile with this set to 0 means any service expecting a genuinely separate low-latency fast channel on that line won't get one, since the line is unified into a single transport class instead.