Well, looking at the subVI, it looks like they're assuming you have a wheel in each corner, oriented at 45º to the frame of the robot.
(thus you can still call them "top-left, top-right, bottom-right, and bottom-left")
Hopefully this context help clears things up:
Magnitude is your speed
Direction is which direction you want to travel
Rotation is how much you want to turn the orientation of the robot.