Originally Posted by TheCyberDruid
if the XML file in it contains infos about the WinUAE config, I see it's use.
As I understand it, it's along these lines... but rather than have information about the WinUAE config itself, it has information about the system requirements for the particular game (e.g. 'this game needs an ECS machine with 1MB RAM').
Then the emulation frontend which loads the RP9 file (i.e. the Amiga Forever 2009 Player) takes that information and translates it into WinUAE configuration settings.
In theory this makes it portable, although in practice as only the Amiga Forever 2009 Player opens them, and WinUAE is the only Amiga Windows emulator worth considering, perhaps you could question the point!