Boxes, for example, have Bottoms and Tops in addition to Fronts and Backs.

Examples where the bottom/top scan is a very important scan:
http://ia902601.us.archive.org/16/items/mbid-ca492a70-9c95-4810-8133-d5de292f29ee/mbid-ca492a70-9c95-4810-8133-d5de292f29ee-9565478309.jpg (box)
http://ia902507.us.archive.org/1/items/mbid-6a5a2335-820d-413c-80fa-960c20e711a5/mbid-6a5a2335-820d-413c-80fa-960c20e711a5-7811781675.jpg (paper slipcase)
http://ia802305.us.archive.org/34/items/mbid-00942351-1eea-4f99-9189-b6c22b1bfa79/mbid-00942351-1eea-4f99-9189-b6c22b1bfa79-8128844215.jpg (regular CD jewel case)