4.6 The reference component (`{reference}`)

The utility of the reference component is to uniquely identify a particular thing. ### 4.6.1 Reference components with type `model` For classes, collections, concepts, properties, and shapes, a human-readable reference component is strongly preferred. This makes it easier to type such IRIs in SPARQL queries. #### 4.6.1.1 Use of camelCase and CamelCase The following conventions are widely applied for the reference component of model IRIs: - The reference component for classes and node shapes starts with an uppercase letter and applies CamelCase. - The reference component for properties and property shapes starts with a lowercase letter and applies camelCase. When CamelCase and camelCase are applied, acronyms and abbreviations are treated as one word. In the absence of whitespace or reading characters, a sequence of two or more acronyms or abbreviations ― while an unlikely construct ― would otherwise become unidentifiable. The difference between using CamelCase for classes and camelCase for properties is often useful because a class and a property that links to instances of that class often have the same human-readable name. The following example illustrated this: ```turtle prefix album: <{scheme}://{domain}/{context}/id/album/> prefix def: <{scheme}://{domain}/{context}/def/> prefix person: <{scheme}://{domain}/{context}/id/person/> album:petSounds a def:Album. album:petSounds def:hasCreator person:brianWilson. ``` Notice how the class (`def:Album`) can be typographically distinguished from the property (`def:hasCreator`). #### 4.6.1.2 Equivalent reference components For node shapes (`shp:`), the `{reference}` component is identical to the `{reference}` component of the corresponding class (`def:`) or concept (`con:`) IRI. For property shapes (`shp:`), the `{reference}` component is identical to the `{reference}` component of the corresponding RDF (`con:`) or OWL (`def:`) property IRI. If a property shape is uniquely connected to one node shape, then the `{reference}` component of the node shape IRI is also part of the property shape `{reference}` component. The following example uses one node shape IRI and one property shape IRI to illustrate this principle: ```turtle shp:Person a sh:NodeShape; sh:closed true; sh:ignoredProperties ( rdf:type ); sh:property shp:Person_age, #... shp:Person_name; sh:targetClass foaf:Person. shp:Person_name a sh:PropertyShape; #... sh:path foaf:name. ```