Newbie questions - Laser cutting background :)

So, I come from a laser cutting AWC608 background and I am thinking of building an OX CNC but I am curious how you get it to 'mill' or if you can. So, I'd like to basically subtractive carve a shape out of a block of wood sometimes in addition to mainly using it for a router. I've had a good look about and I can't seem to see any software that allows you to subtractively layer cnc paths in 3D so the z axis changes mid cut. Am I missing something somewhere? It seems a shame to have that z...

Newbie questions - Laser cutting background