In general terms, the functional specification states what the proposed system is to do, whereas design is how the system is to be constructed to meet the functional specification. However in writing it, some consideration of design issues must take place, to ensure a realistic system is specified.
When you design a product, inside and out, the most important thing is to nail down the user experience. What are the screens, how do they work, what do they do. Later, you worry about how to get from here to there. There's no use arguing about what programming language to use before you've decided what your product is going to do. In this series, I'm only talking about functional specifications.
When it comes to modern theater, stage directions—the descriptive text that appears within brackets in a script—are an important piece of the puzzle. They speak for the playwright when he is not there. They provide details about how the playwright has imagined the environment and atmosphere. They describe critical physical aspects of the characters and settings. Stage directions can also be critical in dictating the intended tempo and rhythm of the piece. Whether they establish a production’s overall tone or elucidate particular actions of characters, stage directions help tell the complete story that is in the playwright’s mind. Stage directions accomplish all of this, using a simple convention that structurally separates them from the actual story.