It is helpful to think of the design process as a sequential mapping between four domains as shown in Figure 3:
1) the mission or customer domain;
2) the functional domain;
3) the physical domain; and
4) the process domain (Suh, 1990).
Decisions made in each domain are mapped into the subsequent domain, moving from “what” to “how” in each mapping, and then zigzagging down hierarchies in each domain as the design is defined in increasing detail. Customer requirements (CRs) are the “what” for the functional domain, functional requirements (FRs) are the “how”. FRs are the “what” for the physical domain, design parameters (DPs) are the “how”. DPs are the “what” for the process domain, process variables (PVs) are the “how”. Results are fed back to the customer, CRs are refined, and the mapping continues. A notional top-level design hierarchy consistent with this scheme is shown in Figure 4.