A Relation is a relation stored as an Identity Attribute of a Digital Subject that links one Digital Subject to another Digital Subject in the same or within a different Context.
Note: If the two Digital Subjects being linked are digital representations of the same real-world Entity then a specialization of Relation called a Correlation should be used.
A Subject Relation is a URI. It is comprised of a ContextId URI combined with an optional identifier string called a SubjectId which if present is preceded by a forward slah, that uniquely identifies a Digital Subject within the Context identified by the ContextId. The SubjectId may only be omitted when the Context identified by the ContextId contains exactly one Digital Subject.
Two kinds of Relations are defined a URIrelation (Hash URI) and XRIrelation. The former is consistent with one of the two proposed W3C's Cool URIs approaches. The latter is an XRI 2.0.
The following ABNF defines a Higgins Relation. Rules not defined in this ABNF are defined in the ABNF for XRI 2.0 (which includes the IRI ABNF from RFC 3987).
relation = URIrelation / XRIrelation
URIrelation = contextURI [ "#" subjectId ]
XRIrelation = contextXRI [ "//" subjectId ]