In the legacy System Engineering processes, the first development phase established a conceptual design for a system. At the end of that phase, the intent was to have identified the characteristic functions of the system, along with other defining non-functional characteristics that distinguish it from other conceptual systems, without having unduly constrained the range of detailed designs that might accomplish the desired objectives.
The concept of a “notional” design came about because some people just can’t handle that degree of specificity.