Not all surfaces are trim surfaces on a model. This makes it hard to tell why you have the number you have. If you create a ruled surface between 2 lines and you just want that surface and build from the edges of that ruled surface it is not a trimmed surface unless you trim something from it like a circle for a hole or a line but is very usable without being a trimmed surface. If you have a surface model that has untrimmed surfaces.Do a analyze test surfaces, check base surface, it will ask if you would like to blank base surfaces if it finds them say yes. the base surface is the original surface before it was trimmed as Mike said. if there is no base surface visable you are probably ok to use the part model.
Check out my surface modeling traing cd at
http://www.cad2cam.net/mastercamx_basic_surface%20.html
Steve
www.cad2cam.net