This is a brief overview of the subset of the OWL language used for defining the PLCS Reference Data and does not attempt to cover the entire language. OWL is an ontology modelling language layered over RDF (Resource Description Framework) that includes the following features:

• XML – provides the syntax for publication and exchange of OWL.
• XML Schema – specifies the datatypes for OWL literal values
• Resource Description Framework (RDF) – a language for relating descriptions to Web resources, whose underlying model is a “Resource-Property-Value” triple where Resources are specified using a Uniform Resource Identifier (URI) and a Value may be a literal or URI. The Resouce-Property-Value triple is formally specified as the Subject-Predicate-Object triple in the RDF standard. Resource-Property-Value is just simpler terminology.
• Resource Description Framework Schema (RDFS) – adds concepts like Class and Generalization RDF for defining simple vocabularies.

OWL then pulls together the elements from these other W3C standards and adds more powerful concepts for defining vocabularies, set theory concepts, individuals and the rest. Because of this layering, when written as RDF/XML the XML Namespaces associated with these other standards appear in OWL XML documents.

Which OWL level is used for PLCS Reference Data?

There are three kinds of OWL specified in the standard: OWL Lite, OWL DL and OWL Full. These allow levels of implementation and serve as a series of (what are essentially) "conformance class" for such implementations. OWL DL builds on OWL Lite, and OWL Full builds on OWL DL. PLCS has, because of practical considerations, elected to use OWL DL. The “DL” in OWL DL is “Description Logic”. There are reasoners and inference engines that understand DL but far fewer understand OWL Full. The software tools supporting OWL can report whether the ontology is DL or not.

OWL concepts

The core concepts in OWL used for specifying the PLCS Reference Data are:

• Ontology – a representation of knowledge about things in the world and their relationships. An ontology also defines a namespace (in both the general sense of a domain within which names are unique and in the XML sense of a URI and a prefix). An ontology is conceptually similar to a model or a schema.
• Class – a concept in a domain. Classes have members and have subclasses and may be related by overlapping. Classes are very similar to sets and many of the concepts in set theory can be applied when creating classes.
• subClassOf – relates a more specific Class to a more general Class. All members of the more specific Class (i.e. the subclass) are also members of the more general Class.
• ObjectProperty – a property that is a relationship between Classes. The domain of an ObjectProperty is a Class that has the property. The range is the Class that is the type of the property.
• DatatypeProperty – a property with a simple datatype. The domain of a DatatypeProperty is the Class that has the property. The range is the XML Schema datatype that is the type of the property.
• AnnotationProperty – metadata used for the management of the Ontology, Classes, etc. (e.g. a version identifier).
• Individual – a member of one or more Classes. An Individual has the properties of all the Classes of which it is a member, including those inherited through the Class hierarchy defined by the subClassOf relationship. Each property of an Individual may be assigned a literal value or may refer to a resource (e.g. Individual).
• import – the mechanism by which one Ontology uses another. Imported Ontologies cannot be changed in the importing Ontology and the entire Ontology is included. Terms from the imported Ontology are distinguished from terms in the importing Ontology by the use of the Namespace associated with the imported Ontology.

How is OWL used to specify typical modelling concepts?

While OWL is similar in nature to other languages such as EXPRESS or UML, it has its own focus. Some of the concepts which modellers use in those languages are not available in OWL.

• Mutually Exclusive Subclasses (EXPRESS 'ONEOF') – owl:disjoint is the relationship used in OWL to specify that a set of subclasses are mutually exclusive.
• Abstract Class (EXPRESS 'ABSTRACT SUPERTYPE') – OWL has no built-in capability to specify that a Class is abstract.
• Complete Set Of Subclasses (EXPRESS 'TOTALOVER') – In order to specify that all of the possible subclasses of a Class have been defined, the superclass is defined to be the Union of the subclasses. This does not take the place of also specifying that the subclasses using the owl:subClassOf relationship.

           <owl:Class rdf:about="#HumanBeing">
             <owl:equivalentClass>
               <owl:Class>
                 <owl:unionOf rdf:parseType="Collection">
                   <owl:Class rdf:about="#MalePerson"/>
                   <owl:Class rdf:about="#FemalePerson"/>
                 </owl:unionOf>
               </owl:Class>
             </owl:equivalentClass>
           </owl:Class>
  
           <owl:Class rdf:about="#MalePerson">
             <rdfs:subClassOf rdf:resource="#HumanBeing"/>
             <owl:disjointWith rdf:resource="#FemalePerson"/>
           </owl:Class>
  
           <owl:Class rdf:about="#FemalePerson">
             <rdfs:subClassOf rdf:resource="#HumanBeing"/>
             <owl:disjointWith rdf:resource="#MalePerson"/>
           </owl:Class>
           

References

OWL Web Ontology Language Guide (http://www.w3.org/TR/owl-guide/)

OWL Web Ontology Language Reference Guide (http://www.w3.org/TR/owl-ref/)

OWL Web Ontology Language Overview (http://www.w3.org/TR/owl-features/)