And suddenly I understand faceted metadata. Sometimes all you need for that final moment of insight is a good example, and Peter Van Djick’s Columbia Guide Site Map is just what I needed. A facet is simply a “flat”, mutually exclusive (at least as far as the XFML specification is concerned) way of categorising a topic—it can be described as a bottom-up method of categorisation rather than the more common hierarchical top-down approach (as seen on the ODP) which seeks to assign all topics as sub-topics of something else. Peter writes in XFML Background and Concepts that
Faceted taxonomies are generally more powerful for websites than classic hierarchical taxonomies—this seems to make a great deal of sense, and it will be interesting to see this demonstrated by XFML in the near future.
Update: Incidentally, IAWiki have an excellent page describing FacetedClassification.