PSVC is Power Supply Volume control. It is a function in the TAS55xx digital modulators. It allows the power supply (PVDD) connected a TAS5xxx power stage to be adjusted by the modulator and used as a volume control.
Switch Mode Power Supplies are recommended for use with the TAS5xxx power stage and it is simple to design a power supply section that can be adjusted by the PSVC PWM output of the TAS55xx modulator. The amount of control is programmable in the modulator but generally 12dB range is the one used. This means that when the modulator controls the system gain, the first 12db will be done by controlling the power supply so the power supply must be adjustable over a 4 to 1 range. After that point, the modulator applies digital attenuation to the input signal to reduce the amplifier's output.
The advantages to this approach are that the amplfier runs cooler because the PVDD power supply is lower and improved audio quality because there is no digital attenuation at "normal" listening levels.