The domain object is used for delegating nameservers for reverse DNS zones. To create one, you should go here and there will be a form to fill in. It should look like the below
The prefix is where you put in the prefix you have rDNS delegation control over in. You don't need to write it in in.addr.arpa ipv6.arpa format, just input the prefix.
YOU MUST DELEGATE THE PREFIX ON YOUR NAMESERVERS BEFORE YOU TYPE THEM HERE.
nserver is where you type the nameservers that host your rDNS delegation. You must have at least two but can use the arrow to add more of them as needed.
Once you've filled in all this information, you can click submit and you've delegated your prefixes rDNS to your new provider!