Currently filled and masking regions only allow for boundary lines, although they typically aren't referred to that way. Regions only permit a single closed loop of lines to create the boundary. Compare to the way linework functions in shafts, where you can create the boundary but also create symbolic lines that are not part of the closed loop condition. Could be very useful in specific cases to avoid making superfluous groups, when it doesn't make sense to go to the effort of making a family. Since this functionality already exists for shafts, it seems like it should be easy to implement in filled and masking regions.