POWER_STATE_TYPE - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTPOAPI_H

typedef enum _POWER_STATE_TYPE
{
    SystemPowerState = 0,
    DevicePowerState
} POWER_STATE_TYPE, *PPOWER_STATE_TYPE;

#endif

View code on GitHub

This enumeration is documented in Windows Driver Kit here and here.