This is the falling-threshold counterpart to hh3cRmonExtAlarmRisingEvtIndex - it names the event-table entry that should fire when the sampled value drops to or below the falling threshold. As with the rising index, zero disables notification for that condition, so an admin who only wants alerts on spikes, not recoveries, can leave this at its default while still setting a nonzero rising event index. Pointing this at event index 4 might trigger a condition-cleared trap once error counts fall back to normal.