When you park a domain in web hosting context, usually within the hosting control panel, it is to have the parked domain having the same content with the primary domain.
e.g. If your primary domain is abc-book.com, and when you park abc-book.NET to your account, abc-book.net will shows the same content as abc-book.com .
It is different from domain forwarding, which is to redirect the url to another url destination. Parked domain will NOT change the domain url display on browser.
Most hosting plan allows unlimited park domain.