Project sign-off usually means accepting the project during the contract closure phase of the project closure, which involves making sure that all the deliverables are there, and all the deliverables are according to the project scope.
Project sing-off means that:
Project is accomplished in time;
Customer is satisfied with the result;
Outcomes meet the project scope and quality;
Project reaches its objectives and can be closed as appropriate;
Project performers are no more responsible for further development without any new billing;
In general, we can say that project sign-off means a formal acceptance of the client, regardless of when this acceptance occurs and why.