What are the objectives of the game?
How does the game flow for the game player?
What are the rules to the game, both implicit and explicit. This is the model of the universe that the game works under. Think of it as a simulation of a world, how do all the pieces interact? This actually can be a very large section.
How does the physical universe work?
If there is combat or even conflict, how is this specifically modeled?
What is the economy of the game? How does it work?
A graphical description of how each screen is related to every other
What is the purpose of each screen?
What are the options and how do they affect game play and mechanics?