From RDF Working Group Wiki
Systems wishing to skolemise bNodes, and expose those skolem constants to external systems (e.g. in query results) SHOULD mint fresh a "fresh" (globally unique) URI for each bNode. All systems performing skolemisation SHOULD do so in a way that they can recognise the constants once skolemised, and map back to the source bNodes where possible.
Systems which want their skolem constants to be identifiable by other systems SHOULD use the .well-known URI prefix.