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
$xml .= " <variable name=\"default_area_code\" value=\"" . $row['default_area_code'] . "\"/>\n";
I'm not even sure what it does in FreeSwitch. It doesn't seem to be referenced by anything other than the config files.
All in all, a strange option. Personally, I'd say stick to using Prefixes on outbound routes instead.