typedef enum _IO_PAGING_PRIORITY
{
         IoPagingPriorityInvalid = 0,
         IoPagingPriorityNormal = 1,
         IoPagingPriorityHigh = 2,
         IoPagingPriorityReserved1 = 3,
         IoPagingPriorityReserved2 = 4
} IO_PAGING_PRIORITY;

Windows Vista Kernel Structures