Ok, I'm going to explain as though you haven't started developing the game yet.
In Functional requirements, u need to write down everything that your game must be able to.
// pseudo example
Functional Req
--------------
- My game must be able to read a CSS file and convert it to an image
- My game must be able to add scores and display it on the scoreboard
- My game must support all kinds of sounds to tell player what he/she is doing with my CSS ...
- My game needs to talk to OpenGL API
- etc...
The summation of all these Must-Dos is what makes up the functional requirement -the behaviour- of your game and usually this list goes hand in hand in with your System design. It is the make-up of your game software.
In non-Functional requirements, u need to write down everything that relates to the technical parts that your game needs to work. Usually, everything in this list would be the minimum requirement of each list item that is needed for your game to work.
// pseudo example
Non-Functional Req.
-------------------
For my game XYZ to run, it minimally needs the following:
- Tablet Samsung min version XXXX (for cost predictions)
- 1 octa core processor running at 1.xGHz (for performance)
- Android min version XXX (for reliability)
- Antivirus XXXXX (for security)
- OpenGL SDK min v4 ...
- CSS version ...
- 3 MB
- 1 SD card
- 1 cow
- 2 birds and a lump o' cheese :)
- ... etc...
So effectively, this list outlays the architecture of your game system
Hope this clarifies it a bit and gets u going;
That's it … all the best ?