This read-only octet string, carried as a varbind in stacking traps, identifies the specific device that generated the notification. Admins use it in trap logs to pinpoint which physical unit in a multi-device stack reported the RTI change or heartbeat issue, since a stack can contain several devices sending independent notifications. A trap arriving with this object set to a particular chassis serial number tells the admin exactly which stack member to go check, rather than the stack as a whole.