The document that fixes what will be built: screens, roles, integrations, acceptance criteria, and explicitly what is out of scope.
It is what a fixed price and a delivery date actually rest on. Disputes at the end of a project are almost always disagreements about something the specification never stated.
We write the specification as part of discovery and keep it readable by the people paying for it — exclusions stated as plainly as inclusions, so approval means agreement rather than a signature on jargon.