SUBSYSTEM_INFORMATION_TYPE - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTPSAPI_H
#if (PHNT_MODE != PHNT_MODE_KERNEL)

typedef enum _SUBSYSTEM_INFORMATION_TYPE
{
    SubsystemInformationTypeWin32,
    SubsystemInformationTypeWSL,
    MaxSubsystemInformationType
} SUBSYSTEM_INFORMATION_TYPE;

#endif
#endif

View code on GitHub

This enumeration is documented in Windows Driver Kit.