x-urn in xml namespace

I am seeing this in an xml file (particularly xml/rdf file)

 <rdf:RDF
  xmlns:geoname="http://www.geonames.org/onto#"
  xmlns:a="http://www.w3.org/2000/10/annotation-ns"
  xmlns:filtr="http://www.machinetags.org/wiki/filtr#process"
  xmlns:ph="http://www.machinetags.org/wiki/ph#camera"
  xmlns:exif="http://nwalsh.com/rdf/ex开发者_JS百科if#"
  xmlns:mt="x-urn:flickr:machinetag:"
  xmlns:exifi="http://nwalsh.com/rdf/exif-intrinsic"
  xmlns:geonames="http://www.machinetags.org/wiki/geonames#locality"
  xmlns:dcterms="http://purl.org/dc/terms/"
  xmlns:places="http://www.flickr.com/places/"
  xmlns:dc="http://purl.org/dc/elements/1.1/"
  xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#"
  xmlns:acl="http://www.w3.org/2001/02/acls#"
  xmlns:skos="http://www.w3.org/2004/02/skos/core#"
  xmlns:foaf="http://xmlns.com/foaf/0.1/"
  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:computer="x-urn:freebsd:"
  xmlns:flickr="x-urn:flickr:"
 >

what does all the 'x-urn' in namespace declarations mean?

I'd guess that they're Uniform Resource Names with an "x-" prefix on the scheme to indicate that they're extensions (i.e. not on the registered list). You'll see "x-" prefix notation for extensions in email headers, HTTP headers, and even MIME types.

Sorry, I don't have a list of authoritative references to back up my guess but maybe it will help anyway.