In this engagement model the ultimate audience engagement is collaboration or contribution. Not everyone in the audience will progress to this stage and some authors may think this undesirable.
Collaboration is not that same as participation. Participation might be passive (reading additional content and exploring the world) or active - voting, sharing, commenting, discussing, Tweeting and so on. Collaboration is adding to the storyworld: writing fan fiction, creating videos or illustrations. It’s providing new content that you, as author, are free to embrace or reject.