It will not be accessible. It is a device generated algorithm to uniquely identify the device, as it is outlined and expected by Apple. You can choose to just use that, or you can go through the registration process to configure a username, and password. You can register at anytime, and if you choose not to you will have the auto-login process available to you.
Keep in mind the auto-generated identifier is unique to the device, and you will not be able to access your account from any other device, except the one you log in with. You will only be able to access you account from multiple devices if you choose to register.