If you want to enable publication for projects, you must select the Enable Publication setting in P6. When you enable publication the Project Service will retrieve and calculate this project. The project will then be part of the Extended Schema. The Project Service will monitor how many changes have been made on the project and how long since the project was published.
Each project service run will write the number of changes on that project to a table. If it crosses this threshold, the project will queue for processing. If this project has any changes, it will be processed. Primavera Data Warehouse will use the Enable Publication setting to determine which projects are pulled into the Primavera Data Warehouse database.