Can someone explain them to me? I don't understand exactly what their purpose is.
I dont know much about rel file's but i do know this .rel's files are like scripts telling the game to load your model stage to load instead of the hard coded stage [If it is hard coded]. So basic's its just telling the game to use an adress on your sd card to a certain stage that it was made for.
A .rel file is if you want to port a stage to another stage. So if a stage was based on Final Destination, but you want it to be on a different stage, then you could use a .rel to do that.