This is the index key uniquely identifying a row in the cmQosFlowPolicerThresholdTable, i.e., one configured threshold rule for a flow policer. An admin uses it to distinguish between multiple threshold rules that might be configured on the same policer, such as separate thresholds on yellow-marked frames versus bytes-in. For example, an admin who set up both a yellow-frame threshold and a bytes-in threshold on the same policer would tell them apart by their differing cmQosFlowPolicerThresholdIndex values.