This question relates to ERS H.1.1 This is the clause that, in short form, says…
If an Official Measurer you are bound by ERS H.1.2 which, where you are in any doubt, requires you to consult with the certification authority before signing a certification control form or signing an affected sail.
The certification authority in the UK for the MYA Adopted classes (FS and RC) is the MYA – please contact the MYA’s Technical Officer.
If an owner, builder or designer you can in the first instance check with a local Official Measurer. He should be able to guide you but cannot provide any official ruling or interpretation. This can only be provided by the relevant authority (IOMICA for the IOM, IMCA for the M, ITCA for the 10R, IRSA for the A Class, and the MYA for the MYA Adopted classes and FS variants of the international classes).