Hi Warren, here's an example of the kind of repair that Andrei is referring to. If you start with a surface like this:



If you then cut the surface with a 2D profile like this:



What you now have is a "trimmed surface":



Trimmed surfaces are part of how NURBS models are structured, the trim edges basically mark which areas of a surface are active and which are cut away. If you then select all those boundaries and hit delete the boundaries will be removed and the full underlying surface will be restored again:




NURBS modeling is based on this concept of a trimmed surface, while sub-d modeling is built on a bunch of individual small elements at all boundary areas, there isn't a kind of larger "underlying" element in sub-d modeling like there is in the NURBS object structure.

- Michael