use @type for classes outside of schema.org

[mbjones]

We will recommend the use of '@type' (or rdf:type) when typing resources to classes outside of the core schema.org vocabulary.

• schema.org/additionalType does not provide the semantics that using '@type' or rdf:type does. '@type' or rdf:type provides proper RDF typing making RDF querying across these data easier withot having to mix checking for rdf:type and schema/additionalType without forcing the harvester to pre-process schema.org/additionalType.
• schema.org/additionalType is defined as a URL and not an RDF resource making the use of prefixed URIs problematic. Prefixed URIs in schema.org/additionalType will not expand to the full URI when harvested. This might be problematic where different publishers use different prefixes ( e.g. a typical issue when prefixing Dublin Core, Dublin Core Terms, DCAT with 'dc', 'dct', 'dcterms' getting mixed usage across publishers)

Considerations

• Google Structured Data Testing Tool throws errors for anything that isn't using the schema.org vocabulary, and this may cause questions for some schema.org publishers
• Benefit to using proper typing makes querying easier by not having to check two fields each time a class type is mentioned in a query.

[ashepherd]

Review any use of schema: additionalType in the guidelines and change to use @type when it references a ontology class

[ashepherd]

• update ADR to be Status: Adopted
• update DataRepository guideline
• update Dataset guideline
• be clearer in the guidelines what will happen with the Google Structured Data Testing Tool (that it tests against Google recommendations).