From the Introduction:
Annotation is a pervasive activity when reading or otherwise engaging with publications. In the physical world, highlighting and sticky notes are common paradigms for marking up and associating one’s own content with the work being read, and many digital solutions exist in the same space. These digital solutions are, however, not interoperable between systems, even when there is only one user with multiple devices.
This document lays out the use cases for annotations on digital publications, as envisioned by the W3C Digital Publishing Interest Group, the W3C Open Annotation Community Group and the International Digital Publishing Forum. The use cases are provided as a means to drive forwards the conversation about standards in this arena.
Just for the record, all of these use cases and more were doable with HyTime more than twenty (20) years ago.
The syntax was ugly but the underlying concepts are as valid now as they were then. Something to keep in mind while watching this activity.