The different levels of analysis and design can be thought of as different descriptions of a problem and its possible solutions.
The relevant space of descriptions has been described (Jackson95) as covering two intersecting sets of attributes. The set Di is a set of attributes of the problem domain, stated in terms of that domain without reference to system design decisions; the requirements statement proper is a set of relations on D, or constraints involving terms from D.
The set M is a set of machine attributes. The intersection of D and M, called S, is, in Jackson's terms, the area of specifications, where attributes exist in both the problem domain and the system design, and where constraints derived from problem domain requirements are expressed in terms of machine requirements. (In terms of the Consumer Report Paradigm of our framework, specifications are constraints on the values of reportable attributes.) Figure C.2.2 illustrates the intersection.
Figure C.2: Domain and machine attributes and specifications