6762 6.2. #2
Description
When a Multicast DNS responder places an IPv4 or IPv6 address record (rrtype "A" or "AAAA") into a response message, it SHOULD also place any records of the other address type with the same name into the additional section, if there is space in the message.
In the event that a device has only IPv4 addresses but no IPv6 addresses, or vice versa, then the appropriate NSEC record SHOULD be placed into the additional section, so that queriers can know with certainty that the device has no addresses of that kind.