To Fill Inner Loops
When you fill an inner loop of edges on a surface selected for gathering it is equivalent to "patching" the base quilt of the Mill Volume. The volume will be built as if there was a smooth surface with no perforations. The Fill option opens the GATHER FILL menu:
All—Fill all loops on a selected surface. Select a surface. All inner loops on this surface will be filled, whether they belong to bounding surfaces or not.
Loops—Select loops to be filled. For each loop to be filled, you have to select only one edge. If you gather using Surf & Bnd, the edges must lie on the bounding surfaces. Select additional bounding surfaces if necessary.