Vcc pin of TVS diode array
Using a VCC rail in conjunction with unidirectional TVS diodes allows the device to ensure the data lines are clamped between the supply voltage and ground. This way, they can adapt to different supply voltages without requiring a separate device for each, and start clamping as soon as the voltage is more than a diode drop outside the rails.

Another consequence of this is that if power is applied to the data line while VCC is unpowered, the TVS will conduct from the data line to VCC, just like an IC's protection diodes. You can reduce this by using series resistors to limit fault current.