RE: UTF-8 validation rules

[Marco Cimarosti]

David Hopwood wrote:
(about range U+FDD0..U+FDEF)
> It's for Arabic presentation forms internal to a rendering 
> implementation,
> I assume (although it's not clear why existing private-use characters
> couldn't have been used for that).

Where could I found more information about this range and its usage in
rendering engines?

Thanks.
_ Marco