typedef struct _HEAP_SUBSEGMENT
{
     _HEAP_LOCAL_SEGMENT_INFO * LocalInfo;
     PHEAP_USERDATA_HEADER UserBlocks;
     INTERLOCK_SEQ AggregateExchg;
     union
     {
          struct
          {
               WORD BlockSize;
               WORD Flags;
               WORD BlockCount;
               UCHAR SizeIndex;
               UCHAR AffinityIndex;
          };
          ULONG Alignment[2];
     };
     SINGLE_LIST_ENTRY SFreeListEntry;
     ULONG Lock;
} HEAP_SUBSEGMENT, *PHEAP_SUBSEGMENT;

Windows Vista Kernel Structures