I am new to DT (using DTPO 1.5.3). When I look at a three pane view, for PDF, Text, HTML items, I see “Classify” and “See Also” buttons at the bottom right corner of the window. However, when looking at a URL item, those two buttons do not appear. Am I doing something wrong, or is this behavior a feature or a bug of the program? 
It’s not a bug, and you are doing nothing wrong. A bookmark is all metadata with no content. The Classify and See Aiso procedures look at content.
BTW:
This will probably change in a future release.