Semantic Extensions

Dear RDF editors,

I am a little confused about "Semantic Extensions" in "RDF Semantics" http://www.w3.org/TR/2003/WD-rdf-mt-20030123/ .

You wrote

Particular uses of RDF, including as a basis for more expressive languages such as DAML [DAML] and OWL [OWL], may impose further semantic conditions in addition to those described here, and such extra semantic conditions can also be imposed on the meanings of terms in particular RDF vocabularies. Extensions or dialects of RDF which are obtained by imposing such extra semantic conditions may be referred to as semantic extensions of RDF. [...] Any name for entailment in a semantic extension SHOULD be indicated by the use of a vocabulary entailment term. The semantic conditions imposed on an RDF semantic extension MUST define a notion of vocabulary entailment which is valid according to the model-theoretic semantics described in the normative parts of this document; [...]
An example of a semantic extension of RDF is RDF Schema, the semantics of which are defined in later parts of this document. RDF Schema imposes no extra syntactic restrictions.

and then you wrote
All interpretations will be relative to a set of urirefs, called the vocabulary of the interpretation; so that one should speak, strictly, of an interpretation of an RDF vocabulary, rather than of RDF itself. Some interpretations may assign special meanings to the symbols in a particular vocabulary. Interpretations which share the special meaning of a particular vocabulary will be named for that vocabulary, so that we will speak of 'rdf-interpretations' , 'rdfs-interpretations', etc.. An interpretation with no particular extra conditions on a vocabulary will be called a simple interpretation, or simply an interpretation. A simple interpretation can be viewed as having an empty vocabulary.

Therefore
An RDF document can be interpreted following different semantic extensions (simple, rdf, rdfs, owl etc.). When a person publish an RDF document on the web, how does he express his intended semantic extension for his document? Can that person publish a document that uses urirefs from the rdfs namespace and adopt an interpretation that is not an RDFS-interpretation? 

I do not completely understand the relationships between the following concepts:
- namespace
- RDF schema
- RDF vocabulary
- Vocabulary
- Interpretation categories (ex. rdf-intepretations, rdfs-interpretations)
- Semantic and/or sintactic constraints
- Interpretation of an RDF vocabulary
- Special meaning of a particular vocabulary
- Semantic extension
- Vocabulary extension (as used in the Web Ontology Language (OWL) Reference Version 1.0)

I would appreciate some more detail in the "RDF Semantics" document.

For example:
- Is a semantic extension a set of semantic and syntactic constraints on the possible interpretation? Is a semantic constraint a "special meaning" of a particular vocabulary? Can a vocabulary have different "special meanings"? Does "special meaning" means "canonical meaning" or "default meaning"?

- Is a vocabulary always represented by an RDF-Schema?

- Does a Semantic Extension always correspond to a paticular RDF-Schema or a particular namespace?

- Have an RDF-schema always a "special meaning" assigned to the symbols of its vocabulary?

- Can an agent use (i.e. reason on) urirefs of the rdfs namespace with a formal interpretation that is not an rdfs-interpretation?

- What does it happen if a person publish a document that uses resources from different RDF schemas that have incompatible interpretations?



Thanks for your attention
Francesco



-----------------------------------------------------------------
Dr. Francesco Torelli
Engineering Ingegneria Informatica S.p.a. - R&D Laboratory
 
Via S. Martino della Battaglia, 56
00185 - Rome (I)
tel.: +390649201415
fax: +390649201340 
e-mail: torelli@eng.it