Hi,
In joomla, you have an option to allow or not the registrations. HikaShop uses this option to display or not the registration part on the right side of the login view of the checkout.
So, if you allow the joomla registration, HikaShop will display the registration screen.
When your users register via HikaShop they will be added to joomla. I believe that CB has a plugin which keeps the synchronization with joomla users so that when HikaShop will add the user information, it will be added to CB as well.
Additionally, in the tab display of the configuration you have the option "registration". This option defines the behavior of the system with the user information. If you set it to no registration, the system won't create a joomla account for the user and will just ask him his email address. If you select "simplified registration", it will just ask his email address and will create a joomla user account automatically, generate a password automatically and send his account details automatically by email. If you select "registration", the system will ask the user all the needed information in order to create his joomla account.