Metadata proposal, links or include

We'll have to discuss the difference between what I would call "standalone" metadata vocabularies and embeddable ones.

Perfect example of  "standalone" metadata vocabulary is ONIX, with its XML ONIXMessage root element and MARC XML, with its collection and record root elements.

But a fine example of "embeddable" metadata vocabulary is the dublin core (with many different serializations, including RDF), so simple that it makes no sense to externalize its properties to an external file.

Also schema.org is an ontology, with several serialization, the JSON-LD serialization being of particular interest. 
Same for BIBFRAME, an OWL ontology, -> RDF triples. Can be made standalone (e.g. in RDF/XML) or a JSON-LD structure -> same as schema.org properties.

And here is the issue: if the manifest is finally expressed as JSON-LD, why exporting extended JSON-LD metadata is a separate file? this is more complex to handle, and doesn't help the author nor the UA developer. 

Therefore I will advocate that extended metadata should be externalized if and only if their formal serialization is not compatible with the manifest serialization format and they can be represented as a standalone file. 

Laurent