In this context, it is perhaps most useful initially to contrast the different types of physical artefact handled by 'archives', 'museums', and 'libraries'.
All physical artefacts are axiomatically unique objects.
But whereas the physical stuff of archives is, or is generally organised to be, absolutely unique, that of libraries has for virtually all library users a uniqueness which is of no interest at all. Unless, that is, the library document has a rarity which then makes it almost, or indeed absolutely, unique. Thus the common practice of combining the management of 'archives' and 'rare books' collections within libraries. But, of course, it is more than that. The explanations frequently act as a link between the object displayed and the 'collection' of which that object is a representative.