RDF.standard_prefixes

You're seeing just the function standard_prefixes, go back to RDF module for more information.

A fixed set prefixes that will always be part of the default_prefixes/0.

%RDF.PrefixMap{
  rdf: ~I<http://www.w3.org/1999/02/22-rdf-syntax-ns#>,
  rdfs: ~I<http://www.w3.org/2000/01/rdf-schema#>,
  xsd: ~I<http://www.w3.org/2001/XMLSchema#>
}

See default_prefixes/0, if you don't want these standard prefixes to be part of the default prefixes.