This is simply the table index that distinguishes one power supply from another in a chassis that may have multiple PSUs installed for redundancy. An admin needs this as the join key for correlating a given PSU's descr, status, and output rows across this and the related power-supply-output table, since a chassis with dual or triple redundant supplies needs to know which specific unit an alarm refers to. For example, uchasPowerSupplyIndex=2 on a dual-PSU chassis identifies the secondary/backup supply, so an alert tied to index 2 tells the admin the redundant unit -- not the primary -- needs attention.