Project and Project Server allow you to take a project offline to work on. If you’re a traveling project manager and need to take the project offline for updating, you must first open the plan when connected to your Project Server. To save it for updating offline, select File Save Offline. Once you’ve done this, the menu selection switches from Save Offline to Save Online when you next look at it. Use File Close to close the offline plan.
To save the file online again, connect to the Project Server that the file originated from. Select the file from your recent files list to open it and then select File Save Online to save the project back to the server.