The name makes it sound like this would be some kind of demo, but demos traditionally don't get full-on ratings listings. So, perhaps this is a standalone product, like (sigh) Dissidia Duodecim Prologus Final Fantasy.
Whatever it is, "Prototype" is right. Judging from the last development update, Mega Man Legends 3 is currently in a very early stage, with some test environments and part of a dungeon. Either this rating was awarded early or Capcom is planning to give people a chance to check out the game well in advance of completion.