How does Sparky construct its .menu files?

[wigwami]

I have Sparky MinimalGUI. I'm hoping someone can give me a  description of how Sparky has put together the FreeDesktop menu files.  For example, which is the primary menu and what other ones does it include? 

I'm asking because I cannot seem to create custom FreeDesktop menu categories, despite having done a lot of research. (See my related post here.)

To be clear: my question is NOT about jgmenu. I'm already aware of how the prepend/append files work.

Thanks so much!