ok create a cube and a plane below it.
Now place an ambient light inside of the cube. Render. You will see your whole scene is lit up and it shouldn't be right?
now create a spotlight and place it inside the cube facing down to the plane. What do you think should happen? Render. You will see that the plane is lit up where the spotlight is.
Now turn on depth map shadows on the spotlight and render. Your scene should go black. This is because you now have shadow calculations which make the light realise that it's inside a box and shouldn't illuminate the plane below.
Technical Director - Framestore
Currently working on: Your Highness