The rdf:ID attribute is somewhat similar to the ID attribute in XML and HTML, in that it defines a name which must be unique relative to the current base URI (in this example, that of the catalog). In this case, the rdf:ID attribute appears to be assigning a name (item10245) to this particular kind of tent. Any other RDF/XML within this catalog could refer to the tent by using either the absolute URIref http://www.example.com/2002/04/products#item10245, or the relative URIref #item10245. The relative URIref would be understood as being a URIref defined relative to the base URIref of the catalog. Using a similar abbreviation, the URIref of the tent could also be given by specifying rdf:about="#item10245" in the catalog entry (i.e., by specifying the relative URIref directly) instead of rdf:ID="item10245" . As an abbreviation mechanism, the two forms are essentially synonyms: the full URIref formed by RDF/XML is the same in either case: http://www.example.com/2002/04/products#item10245. However, using rdf:ID provides an additional check when assigning a set of distinct names, since a given value of the rdf:ID attribute can only appear once relative to the same base URI (the catalog document, in this example). Using either form, example.com would be giving the URIref for the tent in a two-stage process, first assigning the URIref for the whole catalog, and then using a relative URIref in the description of the tent in the catalog to indicate the URIref that has been assigned to this particular kind of tent. Moreover, this use of a relative URIref can be thought of either as being an abbreviation for a full URIref that has been assigned to the tent independently of the RDF, or as being the assignment of the URIref to the tent within the catalog.