The "Additional Whois Signature" is part of the data asked during the registration as a Joker.com reseller.
Those input fields are optional, but if you provide them, they will be used to publish your strings in global whois for your domains.
This way your customers are able to identify you as their domain provider.
The strings you provide will be shown as "Sponsoring Reseller".
Please note: You can provide or modify this data at any time in the Reseller's Area.