[This set of inputs [can be defined] as 1) input which is always the same or is of many types, 2) input which occurs periodically (or very infrequently), and 3) input which does or does not seek to destroy the system. Their rationale for developing the classification was to aid in the definition of steps to be followed in order to find the] solution of the problem of a large-scale or complex system.

p. 302; As cited in: Thomas C. Ford (2008) Interoperability Measurement. p. 146 - System Engineering (1957)