Cat A vs Class 1
Cat A and Cat B are defined in certification rules.
Class 1, 2 and 3 are defined in operational rules.
Class 1 and Class 2 performance require the helicopter to be certified in Cat A (and therefore to fulfil the Cat A engine isolation requirements).
Cat A and Class 1 performance requirements are very similar, with few discrepancies such as the vertical distance to clear obstacles (15ft in Cat A, 35ft in Class 1).