Indexing vs. Classifciation

I have often heard people say that indexing is like adding labels to stuff and classification is like adding stuff to bins. I have never understood this. I mean, when you label a document you also place in a bin; if I index a book with "cats", I've also placed it in the "cats bin"; similarly if I place a book in the "cats bin", then I've also labeled the book with the "cats" term. I couldn't see the difference. Until today. Now I think I get it... Weinberger asks what the difference is between tagging things and thinging tags -- he can't see the difference between dragging a photo to a tag and dragging a tag to a photo. When I read this, I thought, well, in one activity you index the photo (dragging the tag to the photo) and in the other activity you classify the photo in a particular bin. Ah, so that must be the difference between adding labels to stuff and adding stuff to bins; between indexing and classification. Now I think I get it. Whew, just in time for semester start...