Curves > Project Curves (usually required after Curves > Create / Modify Curves) closes any gaps between curve lines and the object’s surface by projecting the curve lines onto the surface.
This function is selectable when a point or polygon object is selected in the Model Manager and when that object has curve lines that are not yet projected.
Several functions on the Curves and Sharpen menus require that curve lines lie directly on the object’s surface, so those functions are not selectable until curves have been projected.