This is the index, 1 through 64, identifying a specific monitored instance within its parent VLLI instance group, allowing up to 64 individually tracked sources per group, and being accessible-for-notify it primarily appears as an identifying field within trap notifications tied to a specific monitored source. An admin correlating a fault trap to a specific physical port or CFM session within a larger monitored group uses this instance ID together with the group ID to pinpoint exactly which source triggered or contributed to the group's fault state. For example, a group configured with an all trigger and 5 member instances would need all 5 distinct instance IDs to report a fault before the group-level action fires.