Introduce type hierarchy, range and domain in section 4

[evanp]

Section 4 "Model" introduces the idea of a type hierarchy with this section:

Every JSON object in an Activity Streams 2.0 document is either an Object or a Link. All other types defined in the Activity Vocabulary, as well as all extension types, are derived from these two base types.

This is not clear to someone seeing a Note in an AS2 document, unless they understand that a Note is a kind of Object, and that everything that is a Note is also an Object.

Does it make sense, in this section, to reference the OWL class structure as the way that AS2 types and properties relate to each other?

[trwnh]

We may want to briefly mention how domain/range/subclass/subproperty inferencing works, since we implicitly expect AS2 processors to understand this.

The practical consideration is that a type may not be explicitly stated by the publisher, but a consumer SHOULD be able to handle the document without failure if they can infer this information via the declaration of some other type or property.

[evanp]

It might be useful to link to the non-normative OWL doc here; it's linked from Activity Vocabulary.

https://www.w3.org/TR/activitystreams-vocabulary/#non-normative-ontology-definition

[evanp]

We could also note that the Vocabulary includes "Extends" information for every type, so you can infer the hierarchy from the doc.