Redactor Redactor 3.x Feature Configuration Dividers
To keep the toolbar clean and easy to use, you can add dividers between buttons. To do so, add
divider in the Toolbar > Toolbar Buttons input in a configuration set.
The divider will be added where you add it in the definition, however some buttons (like table) have a hardcoded position in the list or are always added at the end which may make it work slightly different than expected.
You can use the divider a maximum of 20 times in the toolbar.