typedef struct _PS_CLIENT_SECURITY_CONTEXT
{
     union
     {
          ULONG ImpersonationData;
          PVOID ImpersonationToken;
          ULONG ImpersonationLevel: 2;
          ULONG EffectiveOnly: 1;
     };
} PS_CLIENT_SECURITY_CONTEXT, *PPS_CLIENT_SECURITY_CONTEXT;

Windows Vista Kernel Structures