When creating prototypes, you have to consider depth vs. breadth - do you prototype all of the functionality, but then not go into any specific detail (horizontal prototyping), or do you prototype one or more functions, but in a lot of depth (vertical prototyping).The big decisions that need to be made in the conceptual design stage is what interaction mode/style to use (which will be the most suitable for all types of users, tasks, environment, etc...), what interface metaphor to use and the interaction paradigm (desktop, wearable, mobile). We also need to consider what functions will the system perform - task allocation, what will the system and user do, and how will these functions relate back to each other - temporal ordering - and make themselves available to the user.

After working with the conceptual design, hopefully evaluating it, considering alternatives and refining it, eventually you are happy, or you run out of time, the next stage of design is required - physical design?