Defining the customer's or end user's needs is the first and most important step of the product development process. Vague product specifications are a hotbed for misapprehension, leaving the development process prone to errors.
After the PCB design is ready to be tested, severeal PCBs are produced and assembled. The circuits and chosen components can be tested with the software as a system, and tried into 3D printed enclosures.
Low volume production of the tested devices means making 50-300 devices on full-blown assembly lines, injection molding machines and using the whole production line as if thousands of the devices would be created.