The ETL defines what data is taken from one database, how it is manipulated, and where it is stored. The aim is to have all data required for reports available in the Data Marts. Any required data which is not already in the Data Mart needs to be added to the Data Mart and may also need to be added to the CDW if not already there. When this happens data models are updated and the ETL processes are extended. The idea is to work back through the Reporting data sources until the required data is found. This change process is described further in the chapter 'Customizations and Upgrades'.