The service starts when the project team leaves.
The first support incident I remember clearly was not a technical outage. A client employee needed to complete a routine administrative step after a software installation, but the person who had demonstrated it was no longer on site. There was a manual, and the manual described a screen the employee could not see. I was new to the support team. My instinct was to locate the correct screen. A more experienced colleague asked why our record did not tell us which version the client was using. That was the more consequential question.
A handover succeeds when the receiving team can explain and perform the work without borrowing the departing team indefinitely. I ask people to demonstrate that ability while there is still time to change the arrangement. A signed completion record is useful, but it cannot answer the next operational question for them.