From my experience working at my current company, during the closing phase we have a few tasks that need to be done. Close up any loose ends with our vendors, typically ensure that they have the most up to date designs of the product. Make sure logistics are reviewed and are good to go. Our documentation team will complete various reports like design finalization, installation reports, etc. and update reports and finalization our lessons learned log. Once all documentation is in order, we build up a completed sample and present it to the customer for their approval during a scheduled meeting where we write off any remaining concerns or questions. Once they give their approval then off to mass production.