This object supplies the index - limited to values 1 through 4 - that SNMP traps use to identify which of the chassis's (at most four) fan trays triggered a given fan-related notification. An admin relies on this index inside a trap payload to jump straight to the affected tray without having to correlate other objects, which matters on a chassis with several trays generating similar alarms. For example, a trap carrying notif index 2 tells the admin the alert came from fan tray 2, not tray 1, 3, or 4.