Brake discs left and right

I need to change one disc on the vf1000f , I have a parts bike and the disc in better shape is from the other side. They are stamped with a L and R , these are all original discs with the curved groove from inside to outside , both sides look identical why are they labeled left and right

Ok, I’m a dummy before anyone says it. Yes they are opposites, next question, has anyone used Niche Industries discs? They are crazy cheap but I can’t believe they are any good. $ 65 cad for 2