A good default format for subsections in the Design section is the organization of the Requirements section. Typically requirements are gouped in some way meaninful to the user. Since the design must satisfy the requirements, this may be a natural organization. As a system designer you must satisfy the user's needs for Reliability Repeatability Recoverability Reliability is almost too obvious to mention. A reliable system is not merely the product of extensive testing, but a direct result of a good choice of tools, algorithms, and a realistic appraisal of the environment the system will live in.
Nothing makes a user more nervous than a system that changes behavior inexplicably.
Any operation should give the same results when repeated. If there is an error the user will prefer the error to repeat every time rather than come and go. Every system has bugs and has data subject to corruption from environmental accidents, such as electric power loss. What is usually called 'the user interface', i.e., al the ways the user interacts with the program, can make or break acceptability of the product. Now that you know what you have to do, design it.