The VTT power supply is required to be a push-pull supply, meaning that it is capable of sourcing and sinking current.
The VTT termination resistor attempts to pull the signal to the middle level of the voltage swing. Current flows from either the VTT termination voltage to the driver, or from the driver to the VTT termination voltage. The direction it flows is determined by whether the line is being driven Low or High.
If the signal is driven Low (0 volts), the current flows from the VTT supply to the driver. When driven High, current flows from the driver to the VTT supply. As a result, the VTT supply must be a source-sink type.
A source-sink supply also requires decoupling capacitors. VTT-to-GND decoupling capacitors serve for source currents, while VTT-to-VCCO decoupling capacitors serve for sink currents.