A specification tree shows all specifications of a technical system under development in a hierarchical order.

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.

¹ An assembly level is defined as intermediate level when the subsystem contractor prefers to contract a group of units with complex interfaces to another contractor (for instance the operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 and subsystem application software

Application software

Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...

can be an assembly of the data management subsystem).