Where is the format of .desktop files described (e.g. autostart)?

Question

I am adding programs to autostart by adding files to ~/.config/autostart and everything is working fine but I would like to know what some of the properties mean. When I google this I just see many examples of desktop entries. I am interested in a technical definition. Where is this defined?

I am interested in the general question, but for example what does the following do?

Hidden=true
NoDisplay=true

Answer 1

Here is the formal specification: http://standards.freedesktop.org/desktop-entry-spec/latest/

Hidden=true means to treat the desktop entry as if it had been deleted. NoDisplay=true means not to show the application in any menus, but still treat it as if it is installed, for example to associate a MIME type with it. Setting both at once seems to be redundant, or perhaps undefined.