A domain parking package is automatically created upon registration/transfer of a domain where no hosting package was selected during signup.
Depending on when you registered/transferred the domain over, the name of the parking package will either be Linux Hosting: XXX Small or Domain Parking.
The only functionality of the domain parking package is to allow you to manage the DNS. What is DNS?
If you wish to add the newly registered/transferred domain to one of your existing hosting packages then you might run into an error. We would need to terminate the package which will allow you to add it elsewhere.