typedef struct _HEAP_ENTRY_EXTRA
{
     union
     {
          struct
          {
               WORD AllocatorBackTraceIndex;
               WORD TagIndex;
               ULONG Settable;
          };
          UINT64 ZeroInit;
     };
} HEAP_ENTRY_EXTRA, *PHEAP_ENTRY_EXTRA;

Windows Vista Kernel Structures