Stereotypically, content experts focus on organization and flow. Visual and industrial designers focus on the presentation of the product.
Cognitive psychologists focus on the participant's process and problems over time.
And engineers focus on the implementation of software mechanisms. Not only do these multiple foci overlap, but they are intimately interrelated and ultimately inseparable. It's the point of contact, the boundary and the bridge between the content and the reader. In electronic media, it includes everything from the organization of the material to the layout of the screen, the audification of the space, and the way people use their hands and fingers. This helps to identify problems, issues or questions, and to address them with solutions of appropriate scale.