Rdf Primer / quote 11 of 55
Setting a base URI with XML Base
...relative URIrefs such as #item10245 will be interpreted relative to a base URI. By default, this base URI would be the URI of the resource in which the relative URIref is used. However, in some cases it is desirable to be able to explicitly specify this base URI. For instance, suppose that in addition to the catalog located at http://www.example.com/2002/04/products, example.org wanted to provide a duplicate catalog on a mirror site, say at http://mirror.example.com/2002/04/products. This could create a problem, since if the catalog was accessed from the mirror site, the URIref for the example tent would be generated from the URI of the containing document, forming http://mirror.example.com/2002/04/products#item10245, rather than http://www.example.com/2002/04/products#item10245, and hence would apparently refer to a different resource than the one intended.
To deal with such cases, RDF/XML supports XML Base [XML-BASE], which allows an XML document to specify a base URI other than the URI of the document itself.