This is the index value used in this notification to indicate the system level. This notification-only object tells the admin which specific level, 1 or 2, the accompanying event pertains to, which matters because many IS-IS conditions such as overload state or database status are tracked per level, and without this index a trap about an overload condition would leave the admin unsure whether the level-1 or level-2 database was affected. For example, an overload-related trap carrying this object set to level2IS tells the admin the backbone-level database specifically is the one affected, not the local level-1 area database.