Consider a provides property on Annotation

[azaroth42]

... for when the annotation provides some feature or functionality to the target resource(s), either directly or by using the body resource(s).

For example, a client would benefit from knowing that an annotation provides captions to an AV canvas (or multi-media scene) via the body VTT resource.

This would be parallel to the accessibility property already available on body and target resources, which conveys that the resource has the particular feature already. For example, a video with burned in captions has the accessibility feature of openCaptions.

The initial list of features for provides would be from the a11y vocab list, to mirror the existing property: https://www.w3.org/community/reports/a11y-discov-vocab/CG-FINAL-vocabulary-20230718/#accessibilityFeature-vocabulary

However in the future we could add our own entries to cover new use cases.

An example annotation:

{
  "type": "Annotation",
  "provides": ["closedCaptions"],
  "target": "https://example.org/iiif/canvas/1",
  "body": "https://example.org/resources/video1.vtt"
}

( /cc @nfreire @glenrobson)

[nfreire]

I think this would be a good solution for the use cases identified by the AV Annotations Motivations TSG.
The schema.org vocabulary covers the use cases related with open captions, audio descriptions and transcriptions. The only case that is missing is subtitles.

Comparing with the alternative solution of using the "behavior" property with values "overlay" and "sidebar", this proposal makes it more explicit what kind of functionality is provided by the annotations.