Porcess of Identifying Input and Output Values
First, recognize what data is going to be used as input to system, and what will be output from system. Input and output values are parameters of events between system and the outside world. You must note which Input events that only affect flow of control, such as, terminate, cancel or continue. They do not provide input values. For example, supplier code name, supplier code, rate per unit, product description etc. is inputs to a sales system.