This is the AC line voltage this specific PSU last measured at its input, reflecting the actual voltage being delivered by the PDU or wall circuit feeding a UCS rack-mount server. It's a more direct signal of upstream power-quality problems than InputPower, since a PSU can compensate for low voltage by drawing more current, masking a brownout in the power figure alone. An InputVoltage reading sagging from a normal ~208V toward 180V, even while InputPower stays roughly level, points to an upstream facility power-quality issue rather than anything wrong with the server.