Sign-off is the recorded moment a named decision-maker approves a brief or a deliverable, freezing its content and starting the next phase.
A sign-off has three parts: who approved, what exactly they approved, and when. An email saying "looks good" satisfies none of them reliably.
Digital sign-off on the brief itself — timestamped and attached to the version — removes the entire class of "I never agreed to that" disputes.