CUPS Fax Backend Howto?

[Tilman Schmidt]

Johannes Meixner wrote at 04:29 Tue:
> Traditional PPD file syntax only allows the user to select one
> of a list of hardcoded choices in the PPD file.
>
> Since CUPS 1.3 it supports custom options using an extension
> of the CustomPageSize and ParamCustomPageSize syntax, see
> http://www.cups.org/documentation.php/doc-1.3/spec-ppd.html

I'm afraid I don't quite understand that specification.
How would an actual PPD look like that allowed the user to
enter an arbitrary string through such a custom option?

Thanks,
Tilman