"The Product Owner is also accountable for effective Product Backlog management, which includes:
- Developing and explicitly communicating the Product Goal;
- Creating and clearly communicating Product Backlog items;
- Ordering Product Backlog items; and,
- Ensuring that the Product Backlog is transparent, visible and understood. "
And if that wasn't enough...
“…the entire organization must respect their decisions.”
“The Product Owner is one person, not a committee.”
“Those wanting to change the Product Backlog can do so by trying to convince the Product Owner.”
Sounds like "sole authority" to me.