This value, delivered as part of an SNMP notification when an existence-type event trigger fires, reports what kind of existence change was detected: present (a monitored object newly exists), absent (it disappeared), or changed (something about its existence state was altered). An administrator reads this to understand exactly what triggered an existence-based event alert without having to separately poll the monitored object to figure out what changed. For example, an existence trigger watching for a specific interface row to appear in a table would fire with this value set to present(0) the moment that interface is detected as newly existing.