Single-domain licenses in pro nopCommerce are sub-domain licenses.
If a root domain is entered, the plugin will not work on other sub-domain of a website other than WWW. For example, if you entered yourdomain.com, the plugin will work on www.yourdomain.com and yourdomain.com but not sub.yourdomain.com. Conversely, if you entered sub.yourdomain.com, the plugin won't work on www.yourdomain.com and yourdomain.com.
If you wish to buy the themes / plugins once, and use them on multiple domains, we recommend you purchasing multi-domain licenses. Muti-domain licenses allow the themes or plugins to be used on unlimited websites / domains.