Flight tests (not A320) in recent years identified conditions where engine icing (and possibly airframe icing) can occur outside of the generalised certification boundaries of JAR FAR 25 ‘Annex C’. One area is in very low temperatures, including -40C. A particular problem is with mixed phase icing – a mixture of ice crystals and super cooled water, such that on contact with a surface the water froze acting as glue for the other ice particles. Yet again we learn that the physics of the atmosphere and flight are not as well understood as we thought.
Specific engines / aircraft will have different characteristics and thus different limitations – always stay within the flight manual guidance, but allow some safety margin just in case. Very few aircraft have been flight tested in the range and extent of icing conditions experience during line operations, the certification is based on many assumptions; there is no need for you to be the first to prove the assumptions wrong.