When dealing with complex curves (such as aerofoils), one may see unexpected behaviour when trying to create a closed region. You insert an offset line perhaps (making two complex curves), close off the area with carefully positioned small lines, trim where necessary (normally a useful check in itself), and all seems well. Even when doing a “Select Chain” and checking the sketch relations, everything appears correct and present. Alas, the region cannot be used for an extrusion, loft, or any other similar feature operation, with SolidWorks insisting that the region is not closed. Get around the problem by closing the region with excessive overlapping lines, trim it back, and then it should work. Hope this helps somebody!