This Integer32 (1..65535) value is an arbitrary integer that uniquely identifies one row of the Ethernet flow stats table, per the object's own description. Since a device can carry many subscriber flows each reporting many rows of stats 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 cmFlowStatsIndex to see whether they both came from flow 24 or from two different subscriber flows entirely.