From the announcement:
Have you ever wanted to livetweet a book? I often want to. Yet there doesn’t seem to be a standard way of doing that. You could tweet, for instance, “Oh Captain Wentworth! When will you propose? #JaneAustenPersuasion” but the hashtag isn’t standardized, so some people might be tweeting under #Persuasion and others under #AustenPersuasion. Furthermore, the hashtag isn’t specific enough to make it clear which part of the book you’re tweeting about. If you browse other tweets marked #JaneAustenPersuasion, you might find comments about other parts of the book, which may contain spoilers.
Enter Annotags, a concept for a literary annotation hashtag, or annotag…The first letter is the book code type. Uppercase letters refer to raw codes and IDs, and lowercase letters refer to base 64-encoded codes.
Source: ANNOUNCING ANNOTAGS: A CONCEPT FOR A DECENTRALIZED LITERARY ANNOTATION PROTOCOL | JONATHAN REEVE