Just to add, after reviewing the early historical records of engine related icing, the safety gurus in the industry took note of the variances between manufacturers, AC and engines regarding ambient temperature "limits" in precips (we hadn't known about super-cooled at that early time). Indeed these early limits did vary by 5 degree C. The decision was made to harmonize at +10C to avoid confusing pilots transferring between products. This decision seems to have worked out well where it has been followed and is embraced uniformly