Before revision: For a spacecraft system it has the following levels: System (requirements) specification - generated by customer System (design to) specification - generated by system responsible prime contractor Subsystem specifications - generated by system responsible prime contractor Assembly¹ specifications - generated by subsystem responsible contractors Unit specifications - generated by subsystem (or assembly ) responsible contractors.
After revision: For a spacecraft system it has the following levels: System (requirements) specification - generated by customer System (design to) specification - generated by system responsible prime contractor Subsystem specifications - generated by system responsible prime contractor