A subdomain has its exclusive content and it’s like a second website, but it uses the same domain name. Here is an example for more clarity: If the domain name is www.domain.com, then subdomain names will be www.help.domain.com or www.blogs.domain.com, etc.
An addon domain is a second website and it has unique content, too. It’s not necessary to register the new domain name before its host. If you wish to host www.otherdomain.com, there is no problem. Just do it.
As opposed to a subdomain and an addon domain, a parked domain hasn’t unique content. It is a masked forward to the main domain name of your cPanel account. It’s like a website alias. People prefer parked domains in the following cases:
1) If you need a place to park a domain, that still hasn’t website;
2) If you have few domains, which should lead to the primary domain;
3) If you have errors in spelling of your registered domain name.