Does this help?
Helicopters are certified in one of several groups. For example, JAR classifications are 1, 2 and 3, which are broadly equivalent to the UK Groups A, A(Restricted) and B (see the table below).
These are different from Airworthiness Groups, which dictate how well the airframe stands up to a forced landing. In other words, the terms Category A and Category B (as opposed to Group A or B, or Class 1 or 2) are for certification purposes:
Passengers JAR Class UK AN(G)R
Over 19 1 A
9-19 2 A (Restricted)*
Less than 9 3 B**
Category A means multi-engined helicopters with engine and system isolation as per JAR-27/29, and Flight Manual performance based on a critical engine failure concept providing adequate surface area and performance capability for continued safe flight if an engine fails. In other words, in addition to making sure you have power available (by restricting MAUW) it provides space for rejected takeoffs and landings, and obstacle clearance.
Category B means single- or multi-engined helicopters not fully meeting Category A. They are not guaranteed to stay airborne if an engine fails and an unscheduled landing is assumed, possibly with some damage.
Category A helicopters may operate in Performance Class 1, 2 or 3, but Category B machines may only be operated under Class 3.
JAR Class 1 (Group A) helicopters offer the highest protection for passengers and require no forced landing provisions if the critical power unit fails - the machine can either land within the takeoff distance or continue (safely) to a suitable landing area, depending on when the failure occurs (that is, before or after CDP - see below). Group A helicopters must (with one engine out) clear all obstacles vertically by 35 feet, and climb (after CDP) at 100 fpm to 500 feet, then continue at 50 fpm (1.5%) between 500-1000. They must be able to maintain MOCA in the cruise.
However, there is an exposure time concept, measured in seconds, during which there is no guarantee of a flyaway or safe forced landing. The maximum permitted exposure time is a statistically derived figure, during which the probability of an engine failure can be discounted. The idea is to allow older helicopters to operate while new stuff comes in, and is due to expire (in JAA-land) in 2010.
JAR Class 2 machines, or Group A (Restricted), are slightly more flexible and can operate to a slightly less demanding regime, so you have a wider choice of landing sites. They have a limited exposure (although occupants and third parties must remain uninjured) - and can normally continue safely, except when the failure occurs early in the takeoff or late in the landing, so a forced landing may be required, under conditions that allow it, in terms of weather, light and terrain - those done from elevated pads in non-hostile conditions must be done by day only, otherwise you must abide by Class 1. Otherwise, cloud and visibility must be above 500 feet AGL and 800 m. In other words, up to 500 feet above the site level, you have to be able to see and avoid obstacles. After that, when you are presumed to be IMC, you must meet Class 1 requirements up to 1000 feet above the site, so watch your takeoff weight (the rate of climb should be 50 ft/min net).
JAR Class 3 multi-engined types (that is, Group B, for multis below 2730 kg) may have to make a forced landing, while single-engined types will (some multis share facilities and are not classed as real twins). Public Transport operations must be done in sight of the surface, by day, with at least a 600' ceiling. Minimum visibility is 800m. Class 3 is not allowed in IMC or at night. An engine failure below 100 feet should ensure a safe engine-off landing, so no manoeuvring should be done.
The JAR screen height is 35 feet, for takeoff and landing. There are no distance requirements.
I'm sure if I've got anything wrong someone will tell me!
Phil