typedef struct _HEAP_USERDATA_HEADER
{
     union
     {
          SINGLE_LIST_ENTRY SFreeListEntry;
          PHEAP_SUBSEGMENT SubSegment;
     };
     PVOID Reserved;
     ULONG SizeIndex;
     ULONG Signature;
} HEAP_USERDATA_HEADER, *PHEAP_USERDATA_HEADER;

Windows Vista Kernel Structures