A closed surface is a shape with volume. A sheet metal part is typically not closed. View > Surface controls the display of closed surfaces and complete surfaces.
View > Surface > Closed (deactivates Complete
viewing mode) - limits the display to only the closed parts of an object.
Subsequent operations that break the surface will cause the previously
closed object to disappear.
View > Surface > Complete (the default setting;
deactivates Closed viewing mode)
- reverts the display of the object to all parts, both closed and not
closed.