My understanding is that there are two sets of conditions that call for icing. The temperature is between 10C and a few degrees below freezing [I can't remember the exact figure]. And the presence of moisture. Rain, snow, or fog, obviously. But a substantial cloud cover is also a potential icing issue. Clear skies do not require icing.
There may be others with more knowledge, but that's how I've always understood it.
Thanks,
Scott