Whois.com allows you to point multiple domain names to your Web Hosting package. This enables you to have your website running on multiple domain names, without having to go to the trouble of setting up the site for each domain name.
This feature is referred as Domain Alias on our Plesk Windows Hosting servers and Parked Domain on cPanel Linux Hosting servers.
You can add up to 10 Domain Aliases / Parked Domains for your cPanel Linux Hosting package that you purchase from Whois.com.
There is no such limitation on a Plesk Windows Hosting package.
For each Domain Alias / Parked Domain that you add to your Web Hosting package, you need to also add DNS Address Records to point it to your primary domain name. See details
If you have bought a Windows Web Hosting package for yourdomain.com and want to point yourdomain.net and yourdomain.info to the same website, then you may do so by mentioning yourdomain.net and yourdomain.info as aliases for yourdomain.com.