Still we should help to clear OP's concept. It may be helpful for others who are reading this post.
By clicking icon 'addon domain' you can add multiple domains from your cPanel from a single account.
Parked domains do not contain any content, email address or web pages associated with them.
Most commonly, parked domains are used to hold a domain that is for sale or redirect traffic to another domain.
Here you go to check the difference between parked domain and addon domain Addon vs. parked domains
A subdomain is a subsection of your website that exists as a subdirectory in your public_html folder.