Space Fill Preferences Dialog
When enabled (the default), the largest path in the selection (as measured by the area of its bounding box) is used as the container object, regardless of where it lies in the stacking order. However, there may be occasional cases when the desired container path is smaller than some paths in the filling art (perhaps the filling art will be scaled down). In this case, with the preference turned off, a specific path can be forced as the container object by placing it at the bottom of the selection’s stacking order before clicking the Make button.