Every item in a Product Backlog carries an implicit claim: that building it will be worth more than building something else. Until a Sprint produces a usable Increment and someone reacts to it, that claim is a guess. Scrum's answer to guessing is not better estimation — it is shorter feedback loops.
The Increment is the evidence
This is why the Increment must be usable rather than merely finished. An Increment nobody can react to teaches the team nothing, which makes the Sprint an expensive way to have produced no information. The Definition of Done exists to keep that from happening quietly.
Forecast versus commitment
The exam repeatedly offers scenarios where a stakeholder treats a forecast as a promise. A forecast is a projection from what has been observed; it moves when the observations move. The Sprint Goal is the only commitment inside a Sprint, and even it can be renegotiated between the Product Owner and Developers as more is learned — what cannot happen is the Sprint Goal being abandoned quietly while the Sprint continues.