This holds the numeric index, limited to a range of 1 to 2, implying this chassis model supports at most two power supply slots, that identifies which specific power supply slot a related power-supply trap refers to, letting a management system receiving that trap know exactly which of the two supply bays, not just that a power supply changed state, triggered the notification. Without this index, an operator receiving a power-supply-fault trap on a dual-supply chassis would have no way to tell, from the trap alone, whether it was supply 1 or supply 2 that failed, and would have to poll the chassis separately to find out, defeating some of the point of getting a trap in the first place.