Conceptual Design vs Preliminary Design
Let me begin by saying that while I have an engineering degree obtained in the deep mists of time, my work for the last several decades has entailed writing technical proposals, not "doing" engineering.
I am a little unclear on the boundary between conceptual design and preliminary design. I hazard to guess that conceptual design focuses on enumerating functions, form, and interactions. As such it is somewhat agnostic to an actual design. I believe that preliminary design, on the other hand, entails commitment to an actual design expressed in terms of a set of systems and their interactions (I understand that the real focus of preliminary design is the architecture of the system - how the various subsystems interact).
Any feedback welcomed.