This WebSchemas page discusses a proposal to add an 'additionalType' (or 'type') property to schema.org.
Discussion of additionalType proposal
This is not representative or a complete summary, but should give some quick overview of people's positions.
- R.V.Guha - "My personal preference is to just add an attribute called type (or additionalType) which is samePropertyAs rdfs:type and be done with it."
- Ivan Herman - "As a a consequence, I think that having that additionalType stuff be part of schema makes sense, but this also means that schema-aware tools that use RDF vocabularies should know that this is a synonym to rdf:type. Which is not a nice design but, that being said, this may be to only way ahead for microdata users.
- Gregg Kellogg - could add some rules to the microdata/rdf mapping about multiple types, 'Not that I would recommend that anyone do this against the requirements of Microdata.'
- Martin Hepp - "So adding "type" or "additionalType" property to http://schema.org/Thing seems like a quick, pragmatic yet future-prone approach to me.
- Peter Mika - "My main concern is that schema.org is and should be about the schema, not the syntax. The reason of adding this property is to patch microdata, but if schema.org defines this term, it will be available for all syntaxes (microdata, RDFa, possibly soon OData). In both microdata and RDFa, we would end up with two ways of defining types (using typeof/itemtype vs. additionalType). Standard parsers however will only see one type (the one expressed using the standard mechanism).
- Egor Antonov - "Agreed with Peter. "additionalType" is not a part of the (ontology) schema, but a hack for microdata.", "It's just a superset of microdata features, and it can be implemented in a prettier way than "additionalType" property IMHO"
- Adrian Giurca - "Indeed, it would be enough to allow a list of URLs as value of Microdata @itemtype"
- Jason Douglas - "That's not just a token list... it's an ordered token list. You're privileging the first token as the "frame" as Martin calls it."
- Evan Sandhaus - "ideal solution would be to extend Microdata Syntax to allow for multiple types, ... the need for web publishers to associate multiple types with their metadata is sufficiently urgent that the sponsors of schema.org would be completely justified in introducing an 'additionalType' property."
- Dan Brickley - [ after Microdata DOM investigations], "I just don't think it's feasible, so let's try to do the best we can with 'additionalType'."
- Alexander Botero-Lowry - (on Dan's proposal), "I think this is OK but we need to be explicit that for this to work the primary type (itemtype) needs to be a part of schema.org or the additionalType predicate needs to be fully qualified."
- Stephane Corlosquet - "I'd like to see some markup example of what this discussion would lead to."
- James M Snell - "Activity Streams (AS) has it's own basic type model (and "objectType" property whose value is either a simple label representing known types or an absolute IRI to identify types from other vocabularies. Having a "type" property in the schema.org vocabulary would help us tremendously in bridging the AS model into the schema.org realm. I'd certainly be in favor of introducing it. ("additionalType" works too)."