This is the index key uniquely identifying a row in the cmAccPortQosShaperThresholdTable, i.e., one configured threshold rule for an access port's QoS shaper. An admin uses it to distinguish between multiple threshold rules configured on the same access port, such as separate alarms for tail drops versus replicated bit rate. For example, an admin who configured both a tail-drop threshold and a rate-limited-bit-rate threshold on the same access port shaper would tell the two entries apart by their cmAccPortQosShaperThresholdIndex values.