This value is the instance number identifying this specific failed-call entry among possibly several logged for the same port, per the description, serving as the secondary index into the svcAtmFailedCall table alongside svcAtmFailedCallIfIndex. Because the table is FIFO with a fixed capacity (per svcAtmFailedCallAdminStatus), this index cycles as new failures push out old ones, so it's not a stable long-term call identifier but rather a slot number within the rolling log. An admin polling this table repeatedly needs to track (ifIndex, index) pairs together to avoid misreading a recycled slot as the same failure recurring.