We have to put a shelf mark on the books so that we can shelve the book, but that tells us quite a lot about the subject, and if you start putting those three things together the librarian, as manager of his library, can start to put all this information together — in fact, the computer digests it for him — to give him an overview of how effective his operation is, when he should be buying extra copies, when perhaps he should be thinking of not buying quite so much, or being a little more selective.