typedef struct _SUBSECTION
{
     PCONTROL_AREA ControlArea;
     PMMPTE SubsectionBase;
     PSUBSECTION NextSubsection;
     ULONG PtesInSubsection;
     union
     {
          ULONG UnusedPtes;
          PMM_AVL_TABLE GlobalPerSessionHead;
     };
     ULONG u;
     ULONG StartingSector;
     ULONG NumberOfFullSectors;
} SUBSECTION, *PSUBSECTION;

Windows Vista Kernel Structures