Phase-based maintenance is a new concept introduced in SAP S/4HANA Cloud that allows you to track the life cycle of the maintenance processes using nine predefined phases and sub-phases. These phases are set up by SAP and you cannot change them. However, you can control the transition of phases using phase control codes. Phase-based maintenance is only applicable to the new order types Reactive Maintenance and Proactive Maintenance,which are delivered with the relevant Best Practices scope items. It is not mandatory for breakdown and preventive maintenance, nor can it be used with any existing order type. References: New Phase Model for the Maintenance Processes in S/4HANA Cloud, Phase Model for the Maintenance Process, Maintenance Process Phases, Explaining the Phase-based Process