This is the ending tone index for the same transmit frequency band defined by cvdslMCMConfProfileTxBandStart, together bounding exactly which subcarriers this DMT band uses for transmission. Getting the stop index wrong relative to the start, such as a stop index lower than the start, would define an invalid or empty band, which the profile validation triggered by activating cvdslMCMConfProfileRowStatus is meant to catch before it's applied to a live line. Two adjacent bands whose stop and start indexes don't line up correctly could leave a gap of unused spectrum or, worse, an overlap that the DMT modulation scheme doesn't expect.