This notification-only value is the session identifier carried as a variable binding in packet-mirroring traps, distinguishing one mirroring session instance from another under the same mirror configuration. An admin uses it to track the lifecycle of one specific mirrored session across its start, activation, and end/failure traps. For example, matching juniPacketMirrorSessionIdentifier across a juniPacketMirrorSessionStart trap and a later juniPacketMirrorSessionEnd trap confirms they both describe the same mirroring session's full lifetime.