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

ceditor@tnos.ilc.pi.cnr.it