Today’s conventional tagging mechanism, found on flickr, del.icio.us and youtube, allows users to associate web resources (i.e., documents, photos, videos and audios) with a set of keywords (i.e., tags). This mechanism works well for tagging resources that have a direct accessible URL. However, it falls short in being an effective tool for labeling the “deep” content of those resources — e.g., a partial clip from a youtube video.
M. Melenhorst, M. Grootveld, M. van Setten, and M. Veenstra. UXTV '08: Proceeding of the 1st international conference on Designing interactive user experiences for TV and video, page 31--40. New York, NY, USA, ACM, (2008)