Thanks for raising this concern. I'm not finding any documentation on this feature either, so I've filed a feature improvement with our Document Team to add more insight into this. However, looking at our existing Jive Training materials, I do see this is covered in brief under Discussion Management:
- Branching - this process allows the admin or author to move off topic replies and create a new discussion for them in the same group or space. Any replies to the off topic replies will also be moved.
This falls in line with my understanding of the branch functionality, as we use it frequently in support cases (a custom content type for Jive Community) and can also use it for discussions, as this this very post.
Let me know if you had any other questions on this.