More mud for the pool:
As stated, the Category concerns airworthiness.
The Performance Class determines the outcome after an engine fails, by ensuring that a safe margin exists between what the helicopter is able to achieve and what it is required to achieve.
Helicopters with more than 19 passenger seats, or operating to or from heliports in congested hostile environments, must operate under Subpart G of JAR OPS 3 (Performance Class 1). Helicopters with 9-19 passenger seats may operate under Subpart G or H (1 or 2). Helicopters with 9 or less passenger seats may operate under Subpart G, H or I (1, 2 or 3), although a single-engined helicopter will automatically come under Subpart I (Class 3).
Perf 2 is a mixture of 3 and 1, in that, you are Class 3 until you reach a point where you can maintain Class1 Preformance (the DPATO). This means that you must be visual with respect to obstacles till that point (600 ft cloudbase, 800 m vis), and is why machines such as the 212 still require floats.
Category A helicopters may operate in Performance Class 1, 2 or 3. Category B machines must operate under Class 3. However, Performance Class 1 and 2 helicopters must be certificated in Category A, and Class 3 helicopters may be certificated in Category A or B, but only used in non-hostile environments, to give them more choice of reject areas. HEMS aircraft greater than 5 700 kg must operate under Class 1.
Phil