Here is a brief explanation of the difference between a parked domain and an add-on domain
An Addon Domain is a domain name that points to its folder within public_html and appears as a separate website from your primary domain.
This allows you to have two separate websites on your account. You can have cars.com and then candy.com as an add-on domain. The candy.com would be a completely separate website, and visitors would not know that it is part of your cars.com account.
A Parked Domain is a domain name that points to your main Website on your account
For example, if cars.com is your main website, you can purchase cars.net and assign it as a parked domain so that if a visitor were to type cars.net, they would see the same website as if they had typed cars.com.