This Integer32 (1..32) value is an arbitrary integer that uniquely identifies one row of the Ethernet Access Port history table, per the object's own description. Since a device can carry many access ports each reporting many rows of history data, an admin needs this index to know exactly which row a returned reading belongs to. Two rows both showing high error counts are meaningless until the operator checks cmEthernetAccPortHistoryIndex to see whether they both came from access port 25 or from two different access ports entirely.