The help documentation states:
"Reduces the size of your NX part file when it contains multiple copies of bodies created by the Extract Geometry command."
How is the size reduced?
In a part than has copies of a body (as the result of the Extract Geometry command) only a single copy of the body is saved. In the case of a simple body, such as a sphere, there is not much of a saving. However, when more complicated B surfaces are copied there can be a significant saving.
Does that help?