The null material is not a normal material type (like the exit portal material), but rather indicates that light should pass straight through it, unaffected in brightness and direction.

This on its own is not very helpful, however when combined with the blend material type it becomes very useful for making "cut-outs" such as leaf edges, which would otherwise highly complex geometry.

An example of the null material on its own.

An example of the null material blended with a Phong metal, using a grating texture as blend map.