When the "Copy open files" box is checked, Synchronize! will copy files which are in use by another application.

An example of an open file is a word processing file which is currently being worked on, or a database which is being accessed.

Normally, copying open files causes no problem. However, some files are not in a consistent state unless they are closed. Database files often have this characteristic.

If you work with such files, it is recommended that you uncheck this option, so that they will not be copied if they are open.