Similar to regular User, casual users are those who purchase deal on your site. The main difference between regular user and casual – is that casuals cannot access their accounts on the site.
Casual user is created during fast purchase process. Such users are still visible in Users page of Admin section and their role cannot be changed unless they set up a password for their account and become regular user. All the purchase history and other statistics is being tracked and stored for casual users and they can get access to the account (become regular user) once they set up a valid password.