Intensity Aircraft Ice Accumulation
Trace Ice becomes perceptible. Rate of accumulation slightly greater than rate of sublimation. It is not hazardous even though deicing/anti-icing equipment is not used unless encountered for an extended period of time (over 1 hour).
Light The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the dicing/anti-icing equipment is used.
Moderate The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/anti-icing equipment or diversion is necessary.
Severe The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.
I cannot take your description of icing categorisation in total because light, moderate, or severe would be aircraft specific ie a Boeing 777 would deal with far more icing than I light twin. So icing has to be categorised more by rate of accumulation than by aircraft ability to deal with it. If that was the case icing reports would Vary. ie moderate to a 777 pilot may be severe to a light twin pilot which obviously is not the case in PIREPs
I believe the below is your description
Moderate icing is defined as a level of icing that necessitates de-icing equipment. It really doesn't make sense to have certification of de-icing equipment for "light icing". If the equipment is necessary and can cope with the conditions, the icing is "moderate". If it can't cope with the conditions, the icing is "severe".
Pace