Sometimes it is convenient to be able to assign metadata to the information resources that constitute the occurrences of a topic from within the topic map. To provide this capability, the standard includes the concept of the facet. Facets basically provide a mechanism for assigning property-value pairs to information resources. A facet is simply a property; its values are called facet values. Facets are typically used for supplying the kind of metadata that might otherwise have been provided by SGML or XML attributes, or by a document management system. This could include properties such as “language”, “security”, “applicability”, “user level”, “online/offline”, etc. Once such properties have been assigned, they can be used to create query filters producing restricted subsets of resources, for example those whose language is “Italian” and user level is “secondary school student”.

Note: Once the distinction between addressable and non-addressable subjects had been clarified in XTM, it became clear that information resources could also be subjects (and hence topics). This rendered the concept of facets superfluous since metadata properties can now be assigned to a resource as characteristics of the topic that represents that resource. As a consequence, facets are not part of XML Topic Maps.