The DNS plug-in is displayed when you choose DNS in the Service Kind pop-up menu in the Service Info pane.
This plug-in is used by the default DNS service. It will check a domain name to make sure it can be resolved to an IP address, to detect a DNS server failure.
Unlike the Specific DNS plug-in, this one only checks the default DNS server for your Mac. If you want to use another server, use the Specific DNS one instead.