VSS is the PoE input-power return side. It is the reference for the PoE interface circuits and has a current limited hotswap switch that connects it to RTN. VSS is clamped to a diode drop above RTN by the hotswap switch.
A local VSS reference plane should be used to connect the input bypass capacitor, TVS, RCLS, and the PowerPAD. This plane becomes the main heatsink for the TPS23754.
VSS is internally connected to the PowerPAD.