A domain name resolves to an IP address.
There is no restriction on how many domain names can resolve to the same IP address. If you what complete control of your domain name, you can register what ever domain name you want with a domain name hosting company, then point it to the same IP address.
Lubos posted earlier that the subdomain.accounting.link is designed to be an additional link / domain name. I assume to ensure uniform support of all of the other feature offered by the NG Software cloud product. See About server edition - #2 by lubos and Custom domain - #2 by lubos threads