As best as I can tell, it use the order that they appear in the Registry.
When you add new Scenes, they get Appended to the list in the registry and I believe that also controls the order of Playback.
It scans that list, and if the scene is active that one gets played, if not, it goes to the next one on the list.
I'd have to test again, but that's how it was in the past.
Background 2d used to Always be the 1st one..
Computer\HKEY_CURRENT_USER\Software\Totem\vghd\Scenes