The preserve surfaces option allows user to preserve the surfaces that belong to the object
regardless of the mesh order.
This option is available under the material tab from the edit window of geometric objects.
Preserving surfaces will give users freedom on how to form domains when there is overlap
For instance, here if I preserve surfaces for both sphere and rectangle objects, the
overlapping area between two objects forms a separate domain.
Now, if I choose not to preserve the surface of the sphere, the overlapping area will become
part of the rectangle object.
On the other hand, choosing not to preserve the surface of the rectangle will cause the
overlapping area to become part of the sphere.
This feature can be used to create customized geometries based on your needs.