[yardstick]

It still comes down to librarian interpretation. Sometimes they will just defer to another source, like the national library of their country, or the publishers recommendations, but at least in my experience working part time in a library many years ago, the librarians out back doing the processing and cataloguing would refer to Dewey index guides and also make judgements based on the nature of the book (eg mostly practical vs theoretical nature would be the difference between a 6xx filing and somewhere completely different).