A Null, in Daz Studio, is the simplest form of a node in the scene. What a Null essentially amounts to is a advanced version of a simple marker in 3D space. Where a basic marker would simply indicate a XYZ position in 3D space, a Null Node provides a means of defining that marker's position, orientation (rotation) and scale. A Null Node can also define an arbitrary vector that can be used for automatically pointing (re-orienting) it at another object in the scene, along with providing additional controls for determining whether it is visible and/or selectable within the Viewport.