When talking about 'JAR land' as you put it, you talk about operating in Performance Class 1 or 2 (PC1, PC2), not Cat A. The definitions and requirements for PC1 and PC2 can be found in JAR OPS-3, sub parts F, G and H (and also look at NPA-38 on the JAA web site). Cat A is a certification standard, within which are defined performance standards. Broadly speaking the Cat A performance requirements match those of PC 1.
EMS is still classed as Commercial Air Transport, so the aircraft has to operate to the appropriate performance standard for it's seating capacity. Generally this would require PC2. The other factor most relevant, is the operating environment: is it non-hostile non-congested, non-hostile congested, hostile non-congested or hostile congetsed? Generally a HEMS operation could expect to encounter at least two of those combinations during its operations, and that would also determine the performance standard expected. Likewise, day or night ops.
A brief explanation, possibly with some errors, but it should give you an idea!