2D layers act as impenetrable walls for 3D layers. If you want to create a bug (station identification) that always sits in the lower-right corner, and you don't want anything to ever obscure it, make your bug a 2D layer at the top of your layer stack. That way, no matter how far forward you bring 3D layers in Z-space, they'll never be able to get in front of the 2D layer.

Similarly, if you want a background that is guaranteed to always be the farthest layer back (maybe it's a star field, and you want to make sure that none of the spaceships ever move behind it), keep it a 2D layer and move it to the bottom of the stack.