The system development of spacecraft is divided into four developmental phases, plus an operational phase and - if necessary - a disposal phase, as depicted in the figure below. The system manufacturer, e.g. of an entire satellite or a subunit, usually participates in the first four phases as well as in the start up at the beginning of phase E. Established during this development process are some important milestone reviews with the customer (which for a spacecraft usually is a space agency or a commercial contractor, for a subsystem it is the spacecraft prime contractor).