TABLE OF CONTENTS In these three chapters, we discuss three important parts of a functional system specification: the mission statement, the function refinement tree, and service descriptions. Because you should reach an agreement with the customer about these specifications, they should be made understandable to all.
Chapter 5. The mission statement is a high-level description of the purpose and scope of the system that links desired system functionality to the goals of the composite system. System functionality is circumscribed by stating the system purpose, its major responsibilities, and things the system will not do.
Chapter 6. The function refinement tree refines this description to pieces of functionality, called services, that have a definite starting event and a value for the environment. The root of the function refinement tree represents the system purpose; the leaves represent system services.
Chapter 7. Service descriptions describe these pieces of functionality in language under-standable to both the customer and designer.
The mission statement is the highest-level description of desired system functionality. Whenever you write any system specification at all, whether or not the system is reactive, it should include a mission statement and you should verify it with your customer. Section 5.1 describes the format for a mission statement. Section 5.2 shows how the mission statement should be related to business goals and a business solution specification. Section 5.3 then derives from this the guidelines for writing a mission statement.
The mission statement
