midsurface gap

After extracting the midsurface for a complicated geometry with lots of fillets using rerun option I see some gaps and wired surfaces. Can someone comment on what is the projection direction for rerun option for selected lines (is it normal direction?) Or what  rule  HyperMesh uses in order to select corresponding collapsed line for refilling the gap areas? If you select an enclosed line does it mean that it will project that surface on the gap area? Can plate edit options (e.g. transition surface , plate edge, etc. when extracting midsurface) be used to fill gap areas. Please mention some references for further study.