This reports which severity threshold, if any, the SIOC's temperature readings are currently exceeding. Rather than an admin comparing each of the five zone readings against their configured limits by hand, this single enumerated value already reflects whether any zone has crossed a minor, major, or critical threshold. A value of 'normal' is the common case; 'upper-critical' would mean at least one zone -- most likely rear or VIC under heavy load -- has breached its most severe limit and needs immediate attention.