If a delegating page points to a section that does not exist in the delegate document, this property controls how this fact is resolved.

Which mode you choose depends on your strategy. If you add sections that may or may not exist, you should go with the standard behaviour and suppress the error message. On  the other side, if you require a specific section to exist, you probably want to have the missing section be announced by a visible error message.

Note that an empty section in the delegate document is never considered a problem.