Annotation’s Two Solitudes
Most fundamentally, and despite the fact that the term is frequently used, annotation has not been universally defined. Although annotation can be fairly consistently regarded as editorial metadata, or even information about data, there exists considerable latitude regarding the origin/destination scope of annotation. In the language-centric OWL context, annotations originate internal to the ontology and can annotate internally and to some extent externally. Anecdotally, this remains a common usage mode for annotation as evidenced in the online and off-line literature plus most-mature scientific projects. In contrast in the referencing-based case of XPointer, annotations originate external to the formal ontology, and can annotate the ontology itself or make external references. (The figure below attempts to clarify this not entirely subtle distinction.)
The two solitudes with respect to definitions is carried forward in terms of differences in annotation terminology. As a very concrete example, both the OWL and XPointer communities separately define and make use of the same annotation properties or types. There is clearly an opportunity for the W3C to better harmonize the efforts between these distinct communities, their efforts and common interests.