Transparent object sorting mode of a Camera.
By default, perspective cameras sort objects based on distance from camera position
to the object center; and orthographic cameras sort based on distance along the view direction.
If you're making a 2D game with a perspective camera, you might want to use TransparencySortMode.Orthographic sort mode so that objects are sorted based on distance along the camera's view.
See Also: Camera.transparencySortMode.