Helicomparator:
Like many things, it's not well defined. I know the Canadian FM for the Super Puma was amended to say No flight in freezing Precipitation, not sure about the others.
The logic is as follows:
The clearance for icing is based on testing to the requirements of one of the Appendices to Part 27 and Part 29 (can't remember which letter it is). This is the same wording and requirements as far as droplet size is concerned as the FW world uses. The maximum droplet size used for certification is 0.5mm, and anything larger is considered to be freezing precip. No one has a clearance to fly in freezing precipitation.
Hope that helps!