Pretty much any setting in "default settings" can be copied to "domain settings" then edited as needed.
Select check box next to the setting, then click "copy" in upper right and select which domain to copy to.
It's a switch variable, so I'm not sure if it can. It's not in use though, I think it just got transposed from the default FreeSwitch config but never referenced in the dialplan.
In fact, on further inspection it doesn't even look like it will do anything anyway because of a missing underscore in the variable name:
/var/www/fusionpbx/resources/switch.php