[kovidgoyal]

Just to make sure I understand the isuue. You want a way of locating an arbitrary object in a ebook file (it could be a sentence, a table, a figure etc) unambiguously.

Now we come to the question of resolution. What is the smallest object you are satisfied with being able to reference? A paragraph, (a page at some rendering resolution?). Note that using pagenumbers from printed versions is not good enough as in the future there may not be printed versions.

EDIT: An example from physics research articles. A resolution of sections is usually sufficient. i.e. people refer to section so-and-so of paper so and so.
I don't know if that is sufficient resolution in general though.