cPanel navigates the requests that are sent to the server to the correct account according to domain. Of course, the account owner must declare that the domain belongs to him. In order to ensure that the domain does, in fact, belong to him, cPanel offers two options (without EPP code):
1. To refer the domain DNS to the DNS storage server.
2. To create a randomly-named file on the domain, created by cPanel, which is unique per-user.
I will go into some detail regarding the first option.