In ODF v.1.4 part 3, 19.203 draw:page-number:

> For thumbnails on notes pages, the value of this attribute is fixed to
> the drawing page of a notes page.
>

The wording mentions *a* notes page. This creates an ambiguity, what
does that mean. My initial understanding was, that the value of this
attribute in `draw:page/presentation:notes/draw:page-thumbnail` element
*must* be such, that it points to *its own* parent `draw:page` element.
However, given the ambiguity, it is now less clear, and - however
unlikely I may find it - could the intention be to allow pointing to the
draw page of *any* notes page?

My suggestion is to clarify the wording; and if the value must point to
the own parent, then state it explicitly. Also, it is not stated, what
is the default value of the attribute.