You can modify the geometry of a Structural Member by using another object to create an interference condition within the structural member. For example, you can create a hole in a structural member. Unlike the object used to add a hole, which has no connection to the structural member and can be erased after the hole is added, the object used to create an interference condition remains partially or wholly embedded in the structural member, cutting out and replacing a part of its geometry. All interference conditions are subtractive in this sense, but the option you use to create the interference condition (Add or Subtract) determines whether the outline of the interference condition is included or excluded from the shrink wrap outline of the structural member in plan view.