A type of specification covering an entire complex system, or a major portion of one. As far as I can tell, the term appears to have originated with MIL-STD-490. A good statement of purpose is in DI-CMAN-80008: the A-spec was used to define and partition the requirements allocable to all or part of a system…
“(1) assigned to different contractors or government organizations directly responsible to the contracting agency for that part of the system’ total performance, (2) added on a evolutionary or incremental basis, or (3) planned for major modification.
DI-CMAN-80008 was superseded by DI-IPSC-81431 (now at revision A) but the current DID does less to explain the purpose than did the original DID1.
Footnotes- It also has fewer teeth, e.g. “…The distinction between states and modes is arbitrary. A system may be described in terms of states only, modes only, states within modes, modes within states, or any other scheme that is useful…”.[↩]