This tells the system "If this SPECIFIC USB device is plugged in, make it part of the scanner group with the default permissions (RW by owner and group), and stop processing rules.
Save the file, then:
Save * restart the filehttpd service to pick up the group permissions, and via Apps -> Servers or by hand as root with: service httpd restart* unplug and replug your scanner. to pick up the settings That should be it.