Here are two examples of how you might use functional specifications to measure how well a design performs. Later in the design process, you might test a cross section of riders on a dynamometer and measure how much force they can exert without feeling stressed. You might also measure the grade of the steepest hill on the NU campus. If you are designing a new backpack, you might start with the requirement that it be strong. So the functional specifications help you know what level of performance to shoot for in your design.
Functional specifications evolve significantly over the course of a design project.
Once you narrow your range of design alternatives, however, to a certain kind of solution, your functional specs will become more specific. The functions you'd list for a roller blade solution would not look at all like the functions you'd have for a bicycle. A roller blade solution might specify the time allowed to put on and take off the skates, a specification that would hardly be applicable to a bicycle. Similarly, a bicycle solution would have a function for the time to lock and unlock it from a bike rack. It would be extremely difficult to write initial functional specifications that encompassed two such vastly different design alternatives.