Isn't Y usually up/down?
Think of Z being a 3rd newly added in axis.
Now think of 2d, with X and Y.
Sometimes Y+ is up, sometimes Y+ is north. It depends on the perspective of the game. Mario has Y+ as up, where LoZ has Y+ as north. Adding a third dimension to these two games, using their current Y scheme, would make Mario have Z as into the background, where in LoZ, Z would be up.
So there is no true way to determine whether Y+ should be up or Z+ should be up.