This identifies the specific tone (subcarrier) index for which a Power Spectral Density value is being defined in the transmit PSD table, acting as the index key that pairs with cvdslMCMConfProfileTxPSDPSD to build a per-tone transmit power shaping table. DSL PSD masks are defined tone-by-tone rather than as a single flat power level, letting operators shape the transmit spectrum precisely to comply with regulatory masks or avoid specific interference-prone frequencies. Each row in this table using a different tone index builds up a detailed power-versus-frequency curve rather than one blanket power setting across the whole band.