let me try to explain it easier.
1.when you addning domain to PARK - it will show same website whatever your main domain is showing. in other words this is POINTING into main website.if you park example1.com on example.com, when you type in your internet browser's bar example1.com you will see example.com's website
2 when you add on domain it is mean that on same hosting you can have 2 different websites. adding domain automatically creating a new folder in your file manager, where you will create a new website. if your main domain is example.com and you will add on domain example1.com, when you will type in internet browser's address bar example1.com you will see completly different page, than whatever is on example.com
hope this helps.