Subdomains can not be separated from the domain, because subdomains are part of the domain and subdomains are usually used as a division of the area of a site or website.
Examples for the main domain are "maindomain.com" and separately the subdomain is "my.maindomain.com".
The example concludes that the subdomain is part of the parent domain, and is in front of the domain and separated by dots.
So the word "my" in "my.maindomain.com" is a subdomain and "maindomain.com" is the parent or domain.
At this time we will show you how to easily create subdomains using Plesk.