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This setting is used by the document alert system. Normally, a warning is displayed if an open document has already been previously opened by someone else and has not yet (in Directo's opinion) been closed. This means that if user A opened a document and user B opens the same document, B will see a warning that A has the same document open.
What happens if you make changes to the document and save it in this situation? The user who previously opened the document loses the changes he/she made when saving later. Therefore, the correct behaviour would be not to overwrite the document itself when you see such a message. If necessary, contact the person who has it open.
If the time set here is exceeded, the warning will not be displayed, even though Directo thinks the document is not yet closed. A value 0 disables the warning.