Mission requirements are established in order to help prioritize the work that needs to be done, to identify what functionality is needed, to guide trade off decisions, and to provide a way to verify that the final product meets the mission goals.
These requirements are broken down into three categories:
Level 1 requirements encompass the overall project.
Level 2 requirements encompass the different systems.
Level 3 requirements encompass the individual subsystems.
Content is adapted from Orbital Space Corp's "Orbital Academy Spacecraft Design 2007" presentation.