XFML – eXchangeable Faceted Metadata Language.
>XFML is a model to express topics, organised in hierarchies or trees within mutually exclusive containers called facets. It also expresses indexing efforts: metadata you have assigned to pages. It lets you publish this information in an open, XML based format. Finally, XFML lets you build connections between different XFML maps, by indicating that a topic in one map is equal to a topic in another map: we call this connecting topics, or that a topic is described on a certain resource (a webpage usually), we call this published subject indicators.