The catch-all account is automatically provisioned when the service is created. It is the fallback destination for any email that does not match a specific user in the system.
Once users and aliases have been defined their messages will be delivered to their individual mailboxes. The catch-all is helpful if you have forgotten to create an alias in the system. The downside to simply using a catchall for all of your mail is the intermingling of messages destined for multiple users.
The default catchall account is firstname.lastname@example.org
When your account is created you will get an email with the subject: "Your Email Continuity account has been provisioned" and this will have your login credentials for the admin account.