This is the starting tone (subcarrier) index defining the low end of a frequency band used in this line's DMT multi-carrier transmission scheme, working together with cvdslMCMConfProfileTxBandStop to carve out which portion of the available spectrum this band covers. It matters for spectrum management on DSL lines sharing a binder with other services, since carefully chosen band boundaries can avoid interfering with adjacent services like amateur radio bands or other DSL technologies in the same cable bundle. A provider deliberately notching out a specific tone range here would use this and the corresponding stop index to exclude frequencies known to cause crosstalk with nearby lines.