Flexshaft,
The requirement is established in 29.1517:
Sec. 29.1517
Limiting height-speed envelope.
[For Category A rotorcraft, if a range of heights exists at any speed, including zero, within which it is not possible to make a safe landing following power failure, the range of heights and its variation with forward speed must be established, together with any other pertinent information, such as the kind of landing surface.]
Amdt. 29-21, Eff. 3/2/83
The overall position is discussed in the following paragraph from a 'response' document sent to EASA:
All Category A and B procedures (provided in accordance with Subpart B ‘Flight’ of Part 29) ensure that the helicopter can tolerate an engine failure on take-off or landing: for Category A, by providing profiles that have demonstrated engine-failure accountability; and, for Category B, by providing profiles that remain clear of the H-V avoid curve.
All dual qualified helicopters have a ‘capability’ (as expressed in the EASA definition of Category A) of using the Category A or Category B procedures; helicopters cannot be ‘operated’ in Category A or Category B they have to be ‘operated’ in accordance with the Performance Classes (or with exposure) – i.e. it is the ‘Code of Performance’ that determines which Performance Class has to be employed. Certification in Category A does not, by itself, mandate the use of the Category A procedures.
The issue is complex but if you have the stomach for an interpretation of the issue, it can be found in the 'response' document here: