It's one of the most important and obvious questions from stakeholders and quite difficult to answer accurately.
Using the team's historical completion rate we are able to predict the probable distance down the backlog (using the pessimistic line from the burn-up chart) or the possible distance (using the optimistic line)
The model predicts the 'Probable' and 'Possible' deliverable scope, which helps the business focus on the marginal scope.
There is not much point in focusing on scope around here (1) as this will almost certainly be delivered. Similarly, don't spent too much time talking about scope down here (2) as this is almost certainly not going to be delivered.
Instead everyone should be focused on (3) the scope that *could still be delivered - if the team and 'the business' work through the priorities and manage to improve the throughput rates.